If the others are breathing normally, I would suspect gill flukes (or perhaps a gill infection) making it harder for the affected fish to obtain oxygen. Low oxygen in the water is likely to affect all the fish of a species equally. I would try treating for flukes.
I would reduce the lighting to start with. Brown algae (diatoms) usually go away after a week or two; people can give any advice they like and it will always seem to work. :)
I would also add more plants, especially fast-growing stem plants. Algae seem to have a hard time when there are lots of...
Zero nitrate is not a useful goal.
Happy and healthy fish is a better goal to aim for.
Fish like space to swim around, the choice of interacting with their tankmates or keeping away from them, and a rich environment to live in. No amount of filtration will add that to a tank that is too small.