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Freshwater Stingray / Arowana & big tank filtration related question.     3/15/16
Dear WWM crew,
<Timur>
there might be a bit of text, but bear with me.
<Sure; take your time. Best to be thorough>
I live in Czech Republic and I'm not new to the hobby, but not veteran either, though I do my fish stuff with passion.
<Ahh!>
I'm moving out this year and I had to sell my 120G tank, wood fishes & everything. (Still got 3 small tanks left, which will move with me, red cherry shrimps galaxy Rasbora etc)
Here is my 120G when it was active.
https://www.youtube.com/watch?v=1hWCI6nQoZ8
(https://www.youtube.com/watch?v=1hWCI6nQoZ8 ) 
Initially I wanted to go for Oscars, some silver dollars and bottom dwellers (like angel catfish), but after some weeks of thinking I will go for single silver Aro, school of silver dollars(Metynnis maculatus variant at least 8)
and motoro ray or leopoldi ray (single) on the bottom (depends on the market).
<The Arowana and either Potamotrygonid will grow... to be too large for a 120 gallon tank>
I was thinking to go for smaller variant of FW ray, like reticulatus or hystrix, but after reading I learned, that they are not as hardy(not trying to say any FW ray is hardy here, all are very delicate) as motoro and that the tail gets very long + they are almost impossible to get in my country.
<I see>
I've studied the fish extensively and I'm aware of the diet, sand bottom and the size the fish will grow into. The diseases and issues that these fishes  have (like Drop-eye issue with silver Arowanas) and all the possible information .. dangers of the sting from the ray, treatment and possible long lasting effects. In the end this setup is what I came to.. the tank will be 330 gal/ 1215 liters ish including the filter which will be part of the AQ.
<Oh! Yes>
In ideal situation I would build bigger AQ and put 300liter sump filter below it. But I'm limited by the total weight of the aquarium & all the accessories.
I decided on the fish setup (numbers) in regard to the bioload, surely some people would put more fish in it considering it's a big tank, but from what I studied this tank is a border-line minimum for an adult fish of the
species and I would like it to have as much space there as possible for the fish, without having an empty tank.
<Good; you and I's philosophy is concurrent here>
Since I'm limited by weight (new house, but not meant for x tons of weight concentrated on one spot) .. I decided to put the filter inside (no piping outside the tank, no drilling) here's a diagram. Not here to discuss that, it's going to be inside. No canister filters either.
<All right>
On the diagram you can see that the swimming space for the fish is 200x90x60 which should be enough for these fish considering there will only be 1 Aro, 1 ray and school of smaller fish (15cm max silver dollars).
I will have the water & drainage near the tank so water changes will not be an issue.
<Good>
Pumps will be either Jebao or something similar (intended flow from both pumps choked at 1000L/H each, because any more L/H and the overflow filter would not work properly and would not do the needed bio filtration correctly) adding bunch of Tunze Turbelle to create good flow in the tank.
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I pretty much have a very good idea how the filter could look, here is another diagram, please comment on it.
<Looks very work-able>
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the first chamber is the intake from AQ and there will be a net so the fish wont get in.. now why is it so big you say, well because I want to add some sort of mechanical filtration in there. That's the only thing I'm unsure of how to do properly.. I thought of using the Aquael turbo 2000 internal chamber filter to do that another diagram:
<Open cell foams meant for aquarium use would be my first choice here... I'd have at least two sets; one for in-place use, the other for processing... bleaching possibly, freshwater rinse, air-drying...>
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What do you guys think, or do you have a better solution for this first chamber ? I want to lessen the particles & waste that gets to the sponges area, I do know about the socks that marine aquarists use for their sump
filters, but they clog way too fast and require change every few days according to the marine aquarists.
<Agreed; these bags are better for marine applications>
as for parts of the actual filter: starting from chamber to the left: blue is a sponge which will lessen the sounds that the overflow makes, below it is plastic that will hold it and allow good flow to the pumps (there will be 2 pumps hence 2 pipes inside of the chamber), pumps sit on sponge as to lessen noise.
next mini chamber is 3 cm and is dedicated to create good flow, the 20 cm chamber is purely out of ceramic bio filtration media that will be in a netted bag, so I can easily take it out when needed. We get another small
chamber for flow and then we have the sponges, I am not sure if it should be 20 ppi to 10 ppi left to right (coarse first) or just both of them 20 ppi, because there will be less waste coming.
<Okay>
The big question is how to do the first chamber properly. In ideal solution I would only clean the filter in first chamber i.e. - take out clean, put it back.. once in a few weeks.
<The "blue" sponge material of pore size... I'd switch out weekly, when doing gravel vacuuming, water changes>
And the actual bio media would not get cleaned in months.. all depends on how well the mechanical ones work.. Also what would you put inside of it?
<More foam cut to size... smaller pore size>
Picture related, the chambers are stackable.. up to the height of the tank basically.. I think 3 & 3 would be enough. No sponge there, just that plastic intake.
Sorry for so much text, but it's easier to understand if I write this much
<This is fine; am wanting to understand completely, and be understood in turn>
Thank you for your time
Cheers.
<And you; Bob Fenner>