internal parasite?!

[thechief]

hey guys i recently lost my pbass 5 days after putting a new catfish in the tank and i asked a friend of mine at the store what he thinks killed it and he said it could of been an internal parasite so its got me worried and ever since the pbass died my arowana has been acting strange he is spending 40 percent of the time at the bottom and the rest at the top usually he is always at the top my other catfish is acting strange also he keeps breaking the surface and he never does that i got two new fish today and they are fine but idk its got me really worried that i might loose my aro also the other catfish that i got that i think caused the parasite i had to take back because he was trying to kill and eat my other fish im getting some stuff to treat it tomorrow but im just wondering if you guys think it is an internal parasite or am i just over reacting the arowana is still eating and so is everyone else so i know thats a good sign all normal poop and i also added a very large bubble bar and larger air pump on my tank the other day i turned down the bubbles and it seemed that he got a little less stressed and ive been doing alot of work on the tank but he usually gets over me working on the tank pretty fast the water perimeters are ammonia 0 nitrites 0 and nitrates 15 water temp 82 please help guys