yesterday my male agassizi was acting as you described, i noticed him acting funny and losing his brilliant colour about 3 days ago, so yesterday i did a 33% water change and tested all levels, all ok. but sadly this morning he was dead, sunken to the bottom, standing upright on his nose. that was the first flush funeral i've had to perform in almost a year. sad. i guess the fact i hadn't named him yet, made it a little bit less devastating...
for the record, i had begun using fertilizers for my plants around two weeks ago, with flourish. i use half recommended dose with each water change including the change for the sick agassizi, because i am under the impression it is completely harmless to fish.
there are 30 or so other fish in the tank, including kribensis and my female agassizi (who is now for sale) and all are doing fine, so for now i will continue to use the fertilizer and blame the untimely demise of my unnamed yet treasured male agassizi on spontaneous apisto quietus.
still, i do i really hope it had nothing to do with the fertilizers......
8)