hello, Im pretty new to fish keeping and would some advice on what fish i should add to my tank to have a peaceful 90L/20G tank. Tank currently has 2 synodontis multipunctatus, 3 black phantoms, 3 ottos, 3 platys and 6 neon tetras. There was 5 black phantoms who were all fine for a few months until adding the platys, then 2 got ill and died. I then added 3 clown loaches and 2 died - i returned the last one before he suffered the same fate! I added the tetras a few days ago and one now has a badly injured fin. On researching i think the platys are all male and think they are bullying and being aggressive so these are most likely to be removed shortly. The fish shop people said the aggression/tension is too high in our tank so buy more fish to disperse this. All fish purchases have been discussed with them as well to check compatibility with our tank... Ideally i would like some more colourful fish but been advised against guppies or angel fish as phantoms can be fin nippers. Water tests show water to be fine (dont have exact numbers to hand) Any advice on what could be going wrong or suggestions for further fish?? Thanks!