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.