But having got the tank stabilised as best you can, it’s time to troubleshoot it…

Filtration problems
When more than one fish gets sick in a short space of time, the most likely cause of trouble is water quality. There’s a three-step run-down here that should shake out any possible issues:
Firstly, is the tank overstocked? The old ‘inch per gallon’ rule is a good one, in the sense that’s conservative and allows a bit of space for fish to grow, breed or even for the hobbyist to add a few more fish once the filter is properly matured and running well. It works very well with small fish (Neons, Guppies, and so on) when stocked in tanks 8-10 gallons in size or larger. It doesn’t make sense with anything larger than an Angelfish though. If you’re keeping something like an Oscar, then it’s adult length of around 15 inches definitely does not mean you could keep one in a 15-gallon tank! It’s not a good rule for very small tanks either, anything smaller than 8 gallons not being suitable for ordinary community fishkeeping.
An alternative, though less conservative, rule is to allow 10-12 square inches at the surface of the tank for every inch of livestock. A standard American ‘deep’ 20-gallon tank measures about 24 by 12 by 16 inches, so the surface area is 24 x 12 inches, or 288 square inches. Divide this by 10 and you get 28.8, which is the number of inches of fish you could safely keep in there. That could be fourteen 2-inch Platies, eleven 2.5-inch Corydoras catfish, or some multiple thereof. Because it’s less conservative than the inch-per-gallon rule it doesn’t have the same margin of error, so if you’re using this rule, you should factor in growth and breeding before you fully stock the tank. Also, bear in mind this is another rule that applies to small fish, certainly nothing bigger than an Angel or Gourami.
What happens when a tank is overstocked? As always, reality trumps wishful thinking, and an overstocked tank will see fish dying off until a ‘sensible’ level is attained, at which case things will seem to settle down. Over time those fish will grow or breed, in which case there’ll be another spate of sickness and death. Obviously it’s much better to stock the tank sensibly from the start, not least of all because you’ll save yourself the expense of medicating sick fish or buying replacements a few weeks down the line.
Secondly, is the filter big enough? Aquarists rate filters in two ways, the amount of filter media and the ‘turnover’ rate at which water is pumped through the filter. Most aquarium filters will provide information on these, and there are some rules of thumb that can be handy when choosing a filter. For the average community tank for example, you want a turnover rate of around six times the volume of the tank per hour. In other words, if you’ve got a 20-gallon tank, you want a filter rated at 120 gallons per hour. But to keep things simple, most filters will state the aquarium size they’re designed for on the packaging. These tend to be on the optimistic size, so if you’ve got a well-stocked tank, or you’re keeping fish bigger and messier than Neons and Guppies, then the default filter might be overwhelmed. Either add a second filter, or swap the existing filter for one that’s a bit bigger (carrying across as much mature filter media into the new one as you can).
How do you know if your filter is too small? If your ammonia or nitrite test kit register anything above zero, but your tap water doesn’t contain them, then ammonia and nitrite are accumulating in the aquarium more quickly than the filter can remove them.
Finally, has the filter being properly matured and maintained? Biological filters will take around 6 weeks to mature, sometimes less, but even with the addition of filter aids and bacterial cultures, ‘cycling’ a tank will take a few weeks. Before the filter is mature ammonia and nitrite will rise above zero, and these are toxic to your fish, either poisoning them directly or, at lower levels, stressing them enough to make them prone to diseases such as Finrot. As mentioned earlier, frequent water changes will dilute the ammonia and nitrite. Obviously, don’t add either fish food or new aquarium fish until ammonia and nitrite are back to zero.
Adding ammonia remover (also called zeolite) isn’t a solution here. Chemically removing ammonia will slow down the rate at which filter bacteria grow, and after a few days the zeolite will be saturated anyway, in which case you’re back to square one. What you need to ensure is that the filter has plenty of biological media and that media is properly looked after. Unless you have a specific need to use carbon (and few aquarists do) remove this from the filter and replace it with a good biological filter medium, such as ceramic noodles or sponges. Rinse these out periodically, perhaps monthly, but not under hot water! Ideally, clean them in buckets of aquarium water, or else lukewarm tap water.

Water chemistry problems
Fish are adapted to a specific set of water chemistry conditions in the wild, and while most have some degree of flexibility, too much change can stress or kill them. So far as the casual fishkeeper is concerned, there are three ways water chemistry might cause problems:
Are you trying to keep fish in completely the wrong conditions? If you try to keep Mollies (and most other livebearers) in soft water you’ll probably find that they won’t stay healthy for long. Conversely, species adapted to soft water, such as Neons, tend to be disease-prone when maintained in very hard water. Then there are the brackish water species like Scats and Monos that need some salt added to their water; without it they’ll get sick.
Test your water chemistry (or have your retailer do it for you) and choose fish that are known to tolerate or thrive in those conditions. There are numerous books and websites that disclose the water chemistry tolerances of particular fish, including this one.
Is something in aquarium changing the pH or hardness? Most aquaria tend to become acidic over time, but this is normally a slow process easily offset by doing water changes every week or two. One or two bits of bogwood shouldn’t have much effect, but very large amounts of bogwood relative to the size of the tank can soften and acidify the water much more quickly, which may stress fish unsuited to such conditions. The use of a pH buffer may be necessary if you have soft water to ensure the pH doesn’t become too acidic. Your retailer will have a range of these products on sale, often described as Discus buffer because it’s most often used in tanks with Discus.
Similarly, the addition of calcareous materials such as limestone rocks or large seashells can have the opposite effect, raising the pH and hardness over time. Again, water changes will minimise this, but unless your fish prefer hard water conditions, it’s best to leave such materials out of the tank.
Is your tap water chemically unstable? Some tap water contains a lot of dissolved carbon dioxide, which lowers the pH. This is most common where the water comes from an underground aquifer or deep well. The chemical processes involved aren’t of importance here; what matters is that once the water gets into a bucket, that carbon dioxide will start to diffuse out into the air, especially if the water is warmed up.
How do you know if you have high levels of carbon dioxide in your tap water? Try drawing a bucketful of water tonight, do a pH test on it, leave the bucket somewhere it won’t get spilled, and then do another pH test the following evening. If the pH has risen significantly, then dissolved carbon dioxide leaving the water overnight could be the problem here.
Very soft water is another possible reason for unstable water chemistry. What you want here is a carbonate test kit; anything much below 3˚KH is likely to have very little innate buffering capacity, which means that the tendency of aquaria to become more acidic over time could be exaggerated. If you need soft water conditions for the fish you’ve chosen, then your retailer will have pH-stabilising Discus buffers that will help to fix the pH between water changes. Otherwise, the use of one-half to one teaspoon of sodium bicarbonate per 10 gallons will raise the carbonate hardness sufficiently for a general mixed community tank. What you’re aiming for is carbonate hardness around 5-10˚KH, the lower-end of the range for things like tetras and danios, and the higher-end for livebearers and rainbowfish.

Water temperature problems
Most of the fish we keep in tropical community tanks are happiest around 25˚C/77˚F, which will be a bit warmer than most homes. For this reason a heater is usually required. If the fish are too cold they will be sluggish, disinterest in food, and eventually become sickly and die. Fish that are kept too hot will usually gasp at the surface first, desperate for oxygen as the water warms up, but sooner or later expire from heat exhaustion. It should also be remembered that adding water to the tank that is too hot or too cold will also stress, even kill, your fish.
Do you have a heater at all? Contrary to what some people think, tropical fish can’t be kept at room temperature or under an angle-poise lamp. There are coldwater and subtropical fish out there that may be kept in unheated tanks, but your standard community fish, including Bettas, will need a heater. Your retailer will have a selection in stock, the main thing being to choose one appropriate in size (wattage) to the volume of your aquarium and the coldness of the room. Don’t choose a heater that’ll have to work flat out to keep the tank warm: such heaters will be much more likely to fail. So if you have a 20-gallon tank, don’t choose a heater rated for tanks 10-20 gallons in size, but instead go for the 20-30 gallon option instead. Similarly, if the tank is in a cold basement, get the next size up, because the heater will need a bit more power to overcome the chilling effect of the room.
Is the heater at the right temperature? While aquarium heaters normally have a built-in thermostat with a dial for setting the temperature, the numbers on this dial are only approximate. Use a thermometer to check the actual temperature of the tank, and make slight adjustments to the heater accordingly. The sticky LCD thermometers for example are cheap, reasonably accurate, and very easy to use.
Is the heater broken? Again, a thermometer will help you know if the heater isn’t doing its job, which is why they’re a small but important part of the aquarist’s toolkit. If the water is too cold, the heater might be set too low, but it might also have failed entirely. As with any appliance, check the fuse in the plug before throwing out a seemingly ‘dead’ heater.
If the water is too hot, and turning the dial on the thermostat doesn’t help, then the heater may be jammed ‘on’. This is rare, but does happen, and can end up killing your fish if the water overheats too much. Because of this, it’s always worth buying a heater from manufacturer you trust rather than a no-name heater cheaply available online.
One last thing: condensation inside the heater is not normal, and a sign that water is somehow seeping into the heater from the aquarium. Given these are mains voltage appliances, dodgy heaters should be immediately switched off at the mains, removed and replaced. That said, good quality heaters that aren’t pushed too hard should last for more than ten years.

Problems with poisons and other chemicals
Fish can be exposed to toxins in two ways, either dissolved in the water or in the air above the tank. In either case the aquarist can suddenly find themselves with an entire tank of fish gasping at the surface, rolling about unable to swim or balance properly, or simply dying within a few minutes of exposure.
Has something poisoned the water? Obviously cleaning products such as bleach and detergents must be kept out of the tank, anything used to clean items meant to go in the tank will need to be completely rinsed away. You also want to ensure nothing goes into the tank that isn’t 100% aquarium safe. While it is possible to collect your own rocks and bogwood, you do need to know what you’re doing, and for casual aquarists it is simply safer to rely on material sold in pet shops and garden centres for use in fish tanks and ponds.
Very occasionally things fall into tanks, or are deliberately put in them by ‘pranksters’, that end up doing harm. Anything containing copper, for example, certain coins and screws, will release copper into the water that is highly toxic to most forms of aquatic life, including fish and shrimps.
Has something poisoned the air? Airborne toxins are less of a risk most of the time, but if you’re doing home improvements or a serious spring clean, then this possibility will need to be considered. Some types of paint and many cleaning products contain chemicals that will be stated on the packaging as harmful to people and pets, and these are almost certainly going to be dangerous to fish, too.
Ideally, you’d not use these sorts of products in the same room as the fish, but if that isn’t an option, and moving the aquarium isn’t an option either, then at least minimise the risk by opening some windows and ventilating the room as well as possible. Switching off any aeration devices in the aquarium will minimise contact between the air and water, and turning off the tank lights will trick the fish into becoming less inactive, reducing their need for oxygenated water a bit. It’s also a good idea to throw a sheet or towel over the tank as well, to make sure there’s no chance of droplets of paint or cleaning solvents getting into the water by accident.

Problems with plants
It’s very common for aquarists to have trouble getting certain plants established in their tanks. As with fish, plants have their own requirements, and if you tank doesn’t have the right level of light, or the wrong water chemistry, then those plants will never do well. But what about the situation where a planted tank worked well for years without problems, but suddenly things went downhill?
Have the plants been declining slowly for a while? If your plants were happy until recently, but over the last few weeks or months have grown more slowly or become etiolated, then the chances are there’s not enough light in their world. Fluorescent tubes designed for plant growth having a working life of between 6-12 months depending on the brand. LEDs last many times longer, ten years or more, so while they’re more expensive up front, over the long term they’re actually better value.
The other reason for a slow decline in the health of aquarium plants is the absence of mineral nutrients. Plants suffering this way often turn yellow as they try to move minerals from older leaves into newer ones, essentially recycling iron or magnesium as best they can. Gravel doesn’t contain any minerals that plants can use, so the use of fertiliser tablets will be essential in tanks where gravel is used. But even the high-end substrates designed expressly for planted aquaria will ‘run out’ of minerals eventually. Replacing such substrates isn’t practical without rebuilding the tank, but there’s nothing to stop you adding fertiliser tablets to these substrates should you need to.
Was the decline sudden? Plants tend to react to changes in their environment slowly when compared with animals, but there are some situations where this isn’t the case. For example, Vallisneria and Cryptocoryne species will ‘melt’ when severely stressed, though for different reasons. Vallisneria dislike sudden drops in the pH, being plants happiest in hard water conditions. Cryptocoryne are notoriously sensitive to being uprooted or moved from one tank to another. It may take some weeks for such plants to recover.
Another reason plants can suddenly look in terrible shape is physical damage. If your plants are looking raggedy, it could mean that a fish (or even a snail) has started to eat them. Quite a few commonly kept fish are confirmed plant-eaters, including many of the larger barbs, some cichlids including Severums and Uaru, and most of the medium to large characins, including Distichodus, Anostomus, and of course the Silver Dollars. While most Plecs ignore healthy plants, there are some genera, including Panaque, which will eat aquarium plants. Review your stocklist and act accordingly!

Envoi
Hopefully this gives some insight into the major ways a good aquarium can go bad. If all else fails though, do feel free to write to the Crew here at WetWebMedia and we’ll see what we can do to help!