Speaking as someone who has tap water that reads 30ppm Nitrate, you may take my word for it that I have had some practice with most methods of Nitrate removal. I't so bad around here that a water change is a last resort when it comes to Nitrate removal.
There are a few "nitrate removal Pads/packs" on the market that soak the stuff up and hold on to it - then you "re-charge" the pack by running a salt solution through. They are exceptionally effective - for the first 5 gallons, after which the output readings start to rise very noticably. To generalise price wise - you'd need about ten of them in a seperate filter arrangement, and you'd have to run them for 6 hours a week, followed by a salt solution arrangement to have them ready for the next time. That's ten packs at 10 bucks (plus) apiece. Run them for more than 6 hours and they will have no effect - they'll simply be full up with Nitrate. A single pack has proved pointless at around the 30 min mark in my experience. It's a lot of money - and a lot of work ....... "Nitragon" cannisters are the same stuff - so expect the same results.
Of course, you can go for the disposable industrial packs - if you have a few hundred bucks lying around .......
Anaerobic bacteria ? - yep - you guessed it - been there .... done that. Deep sand beds, substrate additives that promote the growth of the stuf. Not a lot of water flow through a sand bed though - and if you arranged it so it did have a flow, you'd oxygenate the sand bed, killing off the anaerobic bacteria. Catch 22 really. A variation on this theme is the good old de-nitrator. A cylinder or coil with a very low flow rate set up. Hard to set up as a rule, and with such a low flow rate, in your case you'd need more than one. But hey - they cost up to 200 bucks a piece - take 2 months to start working - then you have to fine tune the things ....... again - costs and unreliabilty issues mean they work for some people, but not perhaps for most.
Starting to sound disheartening isn't it ?
The answer as always is very simple ........ Discus keepers have been using it for years. 5ppm Nitrate in a Discus keepers tank is cause for concern, so they use a "total loss water system". Fresh water trickles in at one end of the tank from the water supply - and trickles straight out and down the drain again at the other end of the tank. A permanently ongoing water change basically. Not hard to set up, easy to control flow rates etc. Works like a dream - all you need to do is invest in a bit of extra plumbing, which is considerably less expensive than all those filter packs you'd be chucking away all the time.
Sod science - sod chemicals - just good old fashioned human ingenuity.
Or he could keep less fish of course ........ lol
So Pleco Collector and the others had it right basically - water changes - I'm just suggesting a different approach to dealing with that in such an extreme case.
Have fun *celebrate
-- I hope he has a Python. --
