Honestly some tanks are just cursed. Many people will agree that there's that one awful tank you have at some point that is an absolute disaster no matter what you try. I had a 10 gallon that found a way to kill everything I put in it. I finally tossed it and I haven't had problems with a single tank since, including the few survivors of the cursed tank.
A dog, a cat
5.5 gal: Spike - bicolor HM double feather tail
20 gal tall:4 Pygmy Corydoras, 3 nerite snails, 10 purple harlequin rasboras, 2 apisto bitaeniata
20 gal long: Stormageddon Dark Lord of All- Giant HMPK
SIP Spitfire and Dyson