I have 2 placo. one died?

i have 2 placo who are new to the tank. they were in the same tank before and didnt seem aggressive towards each other. this new tank also has 3 neon tetra a bala shark, nute and goby dragon. so why did he die? should i need alage wafers? its a 30 gallon tank and its filtered very well. i also have a 15 gallon bubbler in there. the ph and ammonia levels are good and the heat is 78 f. there is also aquarium salt in there. i believe the last water change was 3 weeks ago. what should i do to prevent this from happening in the future?