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Reducing Nitrate - Newbie - Freshwater Plenum?

Hi folks,

I tried to make my first post to the list yesterday, but it didn't seem to reach the list, so I'm re-posting (if it did get through and I missed it then please accept my apologies!)

I've been keeping fish for a few months now, so I've still got a lot to
learn!  I've certainly made some mistakes, which is why I am posting this (I've also sent to a fishkeeping magazine here in UK and some newsgroups - I'd like to get several opinions).

The problem that I'm having is with high nitrates.  I have three tanks and one to set up, the tanks are:
48x15x12 - Eheim 2227 Wet/Dry filter and Fluval 4 (internal). 1x42 inch fluorescent tube in a 'marina' hood.  30 inches of fish.
30x18x12 - Interpret Prime 10 and Fluval 3 (internal). 1x24 inch fluorescent tube + reflector  20 inches of fish
19.5inch diameter x 20 inch tall hexagon - Fluval 204 and ClearSeal internal filter (similar to a Fluval 1 or 2).  1 x 12 inch fluorescent tube + reflector and a 'pygmy' lighting unit with a domestic 'energy-saver' fluorescent bulb.  8 inches of fish
48x24x8 - 2xTrio2000 and a Rena iv1 (this tank is not set up yet). 4 x 18 inch fluorescent tubes (2 blue moon, 2 tritons).

All lights are on 14 hours a day (I did have them on 12 hours, but increased the time to try and help the plants.  It has had no effect though).

I don't think the tanks that are populated are overstocked (I anticipate them to be at full capacity once the fish have had a chance to grow).  At present I am feeding them once a day with a small pinch of Aquarian
flake food (though it always seems that they would like more!)

When I started out I saw a product Tetra EasyBalance which Tetra claims to make it unnecessary to do water changes for 6 months if used (it also suggests that Nitrates are controlled).  It hasn't worked for me though (I am still adding it to replace vitamins and so on with water changes though)

Each of the tanks is planted, though the plants do not fare too well, and hair algae has the upper hand.  I am currently fertilising the plants with proAquaPlant from GreenLine aquatics in UK (it's a 'tea-bag' type packet that you place in the water and which is meant to last about 3 months).

Over the last three months I've lost a fair few fish to illness.  It looks mainly like internal bacterial infections (particularly gillrot). 
I did treat this with Interpret remedies and Melafix (followed by water changes at the end of the course of treatment), however the affected fish often died anyway (it was only ever one or two that were ill).  In hindsight, I'm assuming that the illnesses were able to attack the fish due to what must have been excessively high nitrate levels.  At the time I wasn't testing Nitrate,and water changes were infrequent (only after medication) because I assumed that the EasyBalance would work.

At the moment I'm doing weekly water changes (50%) and using rechargable nitrate removal pouches to keep the Nitrate levels down (although there is still a problem with hair algae, and the level is still higher than I'd like by the end of the week).  The fish are much healthier and happier under this regime, but the plants remain fairly unimpressed!

One of the mistakes I made was to just test ammonia and nitrite.  The filters were all working fine and the levels of ammonia and nitrite have remained at 0.  However my Nitrate readings will raise from ~5 to ~100 mg/l in about a week (even with the 50% water changes).

I want to keep the Nitrate levels down more effectively (and if possible reduce the required frequency of water changes).  And I do have an idea which I'd appreciate your comments on.

I'm thinking of setting up a plenum filtration system (like the Jaubert Plenum for Marine fish), with some modifications.

The plenum itself would be constructed from eggcrate and either covered on top with gravel tidy, or something impermeable (in which case the
sides/overhang would be covered with gravel tidy to allow water into the void-space).  The reasoning for suggesting an impermeable top cover is that I understand plant roots will actually oxygenate the substrate, I don't know how much they would oxygenate it or whether they would
disturb the anoxic conditions required for denitrification.

There would be a length of airline tubing running into the plenum so that I could extract water from the plenum if required (in case of hydrogen sulphide production).

I was planning on having an undergravel heating cable either above or below the plenum (I'm unsure which is better, because I assume there must be some water movement in the plenum so that low-oxygen water can enter and so that waste does not build up).

The final addition would be CO2 injection into the aquarium (not into the plenum), to assist the plants.  For this I would plan on stopping
the CO2 overnight, and possibly switching on the airpump during the hours of darkness (since both plants and fish consume oxygen in the dark
- or so I've been told).

I'm hoping that the combined approach (encouraging plants, and using a plenum) will assist in keeping the nitrates lower.  This in turn should result in happier fish, and a better overall look for the aquaria.

I don't have a low-level understanding of the
science behind it all, so I could just be creating a lot of work and spending money, for little benefit to the fish and plants.  Your comments would be greatly appreciated, both on the plenum idea, and the current set-up (I've got substantial conventional filtration on each tank, but from
what I've read recently, it seems as though this may be making the problem worse rather than better  - can you overfilter?).

Apologies for the length of the query and the rambling!  If this is a query you do want to answer, please don't hesitate to contact me for
more information / clarification.

Many thanks for the time you've taken to read this.  I'd really appreciate your comments on the above - even (particularly) if its, 'No, no, no, the existing setup is wrong and the proposed setup is even worse!'

Dominic Large