slowdown

This is such super helpful, specific information - thank you! I'll add more substrate tomorrow and will scrape and move plants as you suggested. Ammonia is 0 so ok for now - I suspected ammonia burn at first, too.

Thank you for your replies - I'm learning a lot. I've had fish for a long time but the planted part is something I did without knowing much beforehand - and now I'm realizing how much I don't know. Are there (cat-safe) plants you'd recommend I add? And when you say the water changes are to reset the nutrient balance, how do you know frequency/amount of water to change - what are you testing to determine what is or isn't out of balance?

Fully agree on not spending for such short-term use. I was dosing 2 pumps at a time - but not consistently every week. So under-dosing could certainly make sense. How do you control nitrates while also giving plants the fertilizer they need? I can do more frequent water changes, but won't that just remove the nutrients I'm putting in?

I haven't added any new fish for a couple of months. The fish in the photo was hanging out near the top and appeared to have buoyancy problems - I thought it was swim bladder but it went from first sign of distress to death in hours. The other three were at the bottom and likewise, went quickly - lethargy for one, the others I just found dead - but all three at the bottom, without bloating, but with the same red splotches either on the belly or behind the gills. The only changes I'd made were to keep the lights off to try to kill off the algae and the Easy Green that seemed to spike the nitrates - though it's possible that the nitrates were already high. I'm very good about regularly checking ammonia - less good about nitrates (lesson learned) - but also not sure why nitrates would be that high with regular water changes. The only other really odd thing with this tank is that I used to have a group of false Julii Cories in the tank and some months ago, all but one died - the remaining one is fat and happy, though solo. I've tried adding more to it twice over the past year so he wasn't alone (once last summer spring, again earlier this fall) and both times all died quickly, other than this one. It's possible the Easy Green/nitrates have nothing to do with the fish death - just was stumped as to other options. Is that the best fertilizer option for this situation, or should I be using something else? If EasyGreen, is the bottle dosing (1 pump/10g weekly) appropriate? If I'm doing more frequent, but smaller, water changes, do I still just dose weekly with the fertilizer? The plants are definitely suffering. I'm not sure what created the algae bloom in the first place but it was pretty severe and the plants started going downhill when that happened.

This is an unheated 20G long that's been running for several years. It has an AquaClear 30 HOB filter and a pre-filter at the intake, plus an airstone. Until last week, it had a dozen white cloud minnows (gold variety) and a half dozen kubotai rasbora. Typically do a 30-50% WC with a Python weekly, filter half that often or less as numbers dictate - it contains a sponge, biomedia, and an ammonia-filter pad. The substrate is sand and fairly thinly layered. Lights are generally on 8-10 hours/day. The problem: I had a large algae bloom last month that took a couple weeks to get under control and since then have lost 4 of the minnows; this after losing an entire school of Cories about a month ago. The rasbora seem fine. I tested the water 3 days ago and was shocked to see the nitrates at 80 (at least - very dark red, hard to say exactly; API liquid test kit). Ammonia was .25 (presumably from a recently dead fish, since removed) and nitrites 0. Did a 50% WC 3 days ago and another today - nitrates are down to around 30-40. PH has been steady at 6-6.5 forever; we are on soft municipal water and I typically keep soft-water fish. Tap water nitrates are 0 (tested 3 days ago). The only changes in the tank are these: -I'd started using EasyGreen per bottle instructions once every week or two -After WC's didn't help with the algae growth (green algae), I left the lights off for 3 days last week. The banana plant leaves look yellowish and droopy and the sword leaves have holes like a potassium deficiency; the grass-like plants (not sure what they are) were thriving until the algae bloom and now are struggling. I've attached a photo of the tank (a couple plants got loose during the WC) and one of one of the fish that died. Any suggestions? Would more plants help the nitrate levels stay low (have to be cat-friendly)? I can keep doing WC's to get nitrates down now, but I don't want this to happen again. I've stopped dosing the Easy Green because of the high nitrates, but thinking that isn't a good long-term option for the health of the plants - but I also don't want to kill the fish. Is there something else I'm not thinking of?