Help: very sick fish

[Debslev]

Hi,
Can anyone help me please? I've got a 1.5 yr old 180l tank that has been fine up til now.
For the last couple of days some of my large catfish have been a little quiet - not running after food etc. but they looked ok and the rest of the tank was fine so I didnt do anything.

Last night came home and I have a definite growth of black fluffy algae; 4 dead neons (no obvious disease; very little bit of wear of tail) and some very sick fish. The cat fish are sitting v still at the bottom of the tank; the red tipped shark is on his side (all looking superficially fine) and my siamese fighter is very low in the tank. This morning another neon dead.

So in the course of 24hrs I've lost all 5 neons; managed to make 3 big catfish and my shark very sick. My plecos seem active but are off food. However, I also have 2 swordfish, 2 angels, 2 cichlids, several guoramis and a black ghost knife fish who do not seem distressed (...yet I fear).

pH is fine and normal (6.5) and I haven't done anything recently I can attribute to this massive problem. (no new fish, no food change...etc!)

Can anyone help suggest (i am going to the fish shop in a couple of hours but right now it is still closed). (In the meantime I did a 15% water change last night)

Thanks,
Debbie

[Lady Hobbs]

Was your tank cycled prior to putting this fish in it? It is most important to check ammonia and nitrites levels. Water changes should be done every week if they haven't been done regularly before this. Fish pee 24/7 and is only removed by water changes.

[Debslev]

thanks for replying. i do cycle my tank. have checked the nitrite levels - at 0; but nitrate is high. I've done 2 water changes, put in a whole pile of "healthy bacteria" for a new tank and left the light off. No more deaths and everything is up the right way but still not looking great.

Neons have all died and massive catfish (15cm) seem to be suffering the most: sitting on the bottom breathing v quickly

Anymore advice would be greatly appreciated