How do I reduce nitrate in an unplanted tank?

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A reader is having problems with the nitrate levels in his tank - Neale Monks offers his advice...

Q) I’m having a problem with high nitrate in my 240 l tank, as my tapwater levels are high. This is giving me issues with algae. I can’t add any plants because they will be eaten by my Silver dollars. I’ve added an external filter with nitrate reducing media, but it is not helping. I don’t want to use RO water because I’m not comfortable about the amount of water that gets
wasted in the process. Do you have any suggestions, please?

PETER HOPPER, VIA EMAIL

A) Neale advises: Nitrate reduction through the use of media is definitely hit and miss. The bacteria involved need low oxygen conditions and creating these in a fish tank — where high oxygen levels are the goal — tends not to work terribly well.

Frequent water changes are the easiest approach, but that does assume an inexpensive source of nitrate-free water. If you live somewhere that isn’t too near a busy road or other source of air pollution, rainwater can be collected safely. The main thing is that your roof is not chemically treated (tiles and slates are fine; things like asphalt likely not) and that the water can run through clean gutters into a suitably clean water butt. Some aquarists choose to filter the water through carbon before use: a small canister filter stuffed with carbon and dropped into the bucket of rainwater does the trick nicely.

Deep sand beds have been used in freshwater tanks and can be very effective, so that’s one option, but you would essentially need to rebuild the tank around such substrates, which may not be viable here. Some bolt-on denitrification filters have been sold over the years, designed to fit into or clip onto the aquarium, but the one or two I’ve tried never seemed to do much. I suspect it’s just unrealistic to assume denitrification to occur quickly enough in a generously stocked freshwater tank to make much difference.

Mechanical filter media can improve clarity but at best their impact on algal blooms will be trivial. On the other hand, UV filters can work wonders in some situations, particularly where blue-green algae and diatom blooms are concerned. But their impact on the large algae types, such as red algae, will be limited. Generally, UV filters are handy in retail situations, where slowing down the rate at which infections can spread throughout interconnected tanks is important. But for home aquarists, their cost is rarely justified.

If money were no object, sticking a sump on the tank and growing plants in there to extract the nitrate would certainly help. Algal scrubbers work in this sort of way and can be extremely effective if set up right.