UPDATE: After doing a 2.5 gallon water change on 21st and then another on the 24th, I figured I'd be in much better shape. I made sure the water was the right GH before adding to the tank. I haven't seen any dead shrimp and there are signs of pending new life, so feeling pretty good.
I have been barely feeding the fish, and I have tons of plants, many of which are floating. Should take care of the nitrates, right?
Dadgummit- I checked the nitrates yesterday and they were at least 30 again. So frustrating!! I did another 2.5 gallon water change and washed out the filter sponge again (just squeeze it out a couple of times in clean treated water). I don't understand what is creating these nitrates at levels that the plants can't eat up. The floaters are red-root floaters, duck weed and some hornwort- they're all growing very well. Should be plenty to take care of it.
I guess "barely" feeding fish is more than I think, although I swear it's a few pellet grains a day and that's it. I'll skip a few days and see....I'm gonna get those nitrates down one way or another.
Edit: I forgot that I did add another slice of zucchini- they at literally the whole thing, so it didn't rot in the tank, but still, maybe that's a source. They will not be getting their veggies for a few days!
How many fishes do you have in there ? Fishes respiration is the major producer of ammonia alongside waste decomposition.
If your Fishes are having a good high protein diet, They will produce more ammonia breaking down these protein to energy.
Do you have some dead spot in your tank ? Where mulm is accumulating ? Most of the time under or close to the filter intake and in the corners are obvious places.