As I've noted in other posts, I have an 80 gallon fish tank, a 110 gallon turtle tank and an ADF/snail tank all with high nitrates. All of these tanks have filtration systems that use biological nitrification, which is of course the source of the high nitrates. Both the fish and turtle tanks have an MBBR, which because of higher efficiency also generate higher levels of nitrates, and the ADF/snail tank has an Eheim canister. The fish tank has both my experimental helical scrubber and my new DIY upflow scrubber, and the turtle tank has a waterfall scrubber. The waterfall scrubber grows lots of algae but the helical scrubber not so much. The new upflow scrubber in the fish tank looks promising.
I have tried multiple methods to try to reduce nitrates with varying degrees of lack of success. I have found that the only method that actually reduces nitrates are a series of several high % water changes (75-80+% each) over the course of a few days or a week. Small, periodic water changes don't accomplish squat, even in a small tank. But big water changes bring their own set of issues. And for me, big water changes in the fish and turtle tanks are not a viable option. Even big water changes must be done repeatedly as long as biological nitrification continues in the filtration system.
I also have a standard gourami 15 gallon tank and a dwarf gourami 2.5 gallon tank with algae scrubbers only and no nitrates. I also have a sponge filter in the standard tank that I sterilize 2 or 3 times per week to prevent it cycling. There are two SM Drop1.2s in the standard tank, and a small DIY horizontal scrubber in the dwarf tank (an Aqueon Bowfront beta tank). So the following idea occurred to me.
Get an algae scrubber up and running, growing lots of algae, then take the nitrifying filter offline. Assuming that algae will consume ammonia first, then nitrites, then nitrates, one might expect that after a transitory ammonia spike lasting a few days, the algae would consume the ammonia and then start consuming the nitrates. Over a period of time, without the constant addition of new nitrates from a nitrifying filter, nitrates should fall to zero. From that point onwards, the algae would live on the daily ammonia production in an equilibrium state where the amount of algae growth is dependent on the daily ammonia output in the tank. The initial ammonia spike could be dealt with in several ways but should eventually go away, as it did in my standard gourami tank after about a week.
I wonder whether anyone else had thought to try this. If so, what was your experience? Just to be clear: I'm talking about taking an established fresh water tank with a cycling filtration system and high nitrates to algae scrubber only. The algae scrubber would be producing lots of algae before taking the nitrifying filtration offline.
Starting a new system with pure H2O, I would avoid nitrification-type filtration and get an algae scrubber established before adding livestock. Just feed the algae pure ammonia and back off when the livestock are added. It's really too bad that the hobby seems locked into nitrification filtration and perpetual water changes to deal with the resulting nitrates. No one ever tells the newbies that there are better ways to keep the water healthy. Once you get beyond a single small tank, the work grows exponentially.
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