Extremely High Nitrates. Fluval Vicenza 190ltr, With Fluval 206 External.

The easiest way to get the nitrates down is by water changes.  But since it is so high, you need to do small frequent water changes till you get it low. Or long term by having a lot of well growing plants in the tank. Nitrates get high for several reasons: not enough water changes; water changes that are too small; too many fish; over feeding.

With the fish you have, you are not overstocked (though your knifefish and clown loaches will need a much bigger tank than 190 litres)
How much do you feed? A rough gudeline is the amount of food equal to one eye per fish. A fish's stomach is around the same size as its eye. Though this applies mainly to flake/pellets rather than live food. Uneaten food decomposes to make ammonia, which is converted to nitrate by the filter.
How often do you do water changes and how big are they? The starting point should be 25 to 30% a week. Then the nitrate reading can help you fine tune that regime. Nitrate should not be allowed to get more than 20ppm above the level in the water you put in the tank. For most people, that is their tapwater level but as you use RO, you should check the level in that before adding it. Do you use pure RO with added remineralisation salts, or do you mix the RO with tapwater?


Assuming the nitrate level is high becasue you haven't been doing enough water changes, there will be other things going on with the water in the tank. The pH will probably be lower than the new water as nitrate is acidic and a lot of nitrate drops the pH. There will be other chemicals secreted by the fish that have built up. A large water change will alter the chemistry of the water too much and the fish will suffer. That's why you need to do small daily water chages till you get the nitrate level to less than 20ppm higher than the water you are adding.


As for changing the filter media - don't. Not unless the ceramics are crumbling and the sponges disintegrating. You can change carbon - but change it for ceramics or sponge not more carbon as you don't need it. But if you have a lot of carbon, only change a bit at once with at least six weeks betwen batches. That also applies to crumbling ceramics and disintegrating sponges. If they are still in good condition, all that's needed is washing gently in old tank water that you take out during a water change.

Don't increase the water changes until you have brought the nitrate down by doing daily small changes. There will be other chemical changes in the water brought on by doing water changes just once a month, and changing too much water straight away will change the nature of the water too much in one go and potentailly harm the fish. Once you have the nitrate level down, then start doing weekly 30% changes.