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So one of the largest problems I've been facing so far is how to sustainably remove nitrate from my tank. I've tried the water changes, but it seems too demanding for a small to no visible effect.

Now what I've tried doing is to grow anaerobic bacteria. I planned on doing this by adding a 1"sand substrate.

My tank is bare now, but I used to have sand, and I must say, it was cleaner back then.

However, I did do some reading and I read that hydrogen sulfide could be produced and it could dissolve into the water, which is bad. But that only happens if it is able to collect. I know my turtles will play in the sand to no end if I put the sand back, but I'm still concerned about the hydrogen sulfide.

Thoughts or comments will be much appreciated. Also any thing I left out or got wrong would also be appreciated if it was pointed out!

Thanks to all!

I do not think it will be an issue with 1" of substrate. I'm not an aquarist but you should be doing larger water changes than the average aquarium and aerating the water. Keep a lookout for any unusual odors and you should be fine.

It's not likely to harm your turtle, but it's also not likely to have a significant effect.

I've read some of the same articles. I have a unique setup with a 55 gallon tank for my turtle, and a 55 gallon tank as a refugium. Slightly more than half of the refugium is dedicated to plants and shrimp, plus snails that can't be fed to the turtle because their shells are too hard.
The main tank has just over two inches of sand. The refugium has closer to four, with the intentional idea of getting anaerobic bacteria to reduce the nitrate to nitrogen that would leave the system. The deep refugium sand has definitely shown the black regions that indicate hydrogen sulfide producing bacteria.

That did like.y mean a reduction in nitrate growth, but it still grew quickly to the point of water changes. I didn't get an appreciable nitrate reduction until I added floating plants.