Sorry, this is a sad crappy post just to get something off my chest.
i had the awful job of removing the bodies of a couple of very much loved fish from my daughters tank just now. A hoplo catfish we called Fred: a big guy that we've had forever and a clown loach. They're the latest victims of something that has gone terribly wrong with the tank in the last ten days.
The thing is i definately think it's something I've done. When we came back from holidays the tank needed a good clean, there was a thin film of algae on the glass and the water level had dropped from evaporation. So I did a water change as usual, added salts, cleaned the glass, changed the filters(all stuff i do every 6 weeks for the past two years). I ordered a new light from the pet shop and picked up a few bristlenose cats to add to the tank to control the algae.
Some days later i noticed Fred hanging about the top of the tank, hiding behind the filter, which he only does if he's being bullied. But there were no fish bullting him. He was also less reluctant to feed. i though the tiny bristlenoses might be upsetting him because they were new and were bottom dwellers also so I thought a few more days with them would calm him down. Then some of the other fish seemed to loose interest in feeding. Then we started having deaths. Both juvenile mollies. I took a sample a water sample to the aquarium to be tested were i found out that the hardness was very high and there was more nitrate then there should be. (I won't go into the ammonia/ nitrite/nitrate cycle only that all are lethal for fish but bacteria in the gravel and filter system breaks one down into the next. You can't break nitrate down but it's also the least deadly to fish so it can be removed with regular water changes to constantly dilute it.)
High hardness is also deadly for fish. the problem is geelong's tap water has suddenly got harder, probably due to the drought and Barwon Waters management of it. I didn't know this and added the usual amount of salt to the water which hardens soft water as you usually need to to maintain the correct ph. level. (Okay, I'm not going into that... just a steady ph. is essentual for healthy fish). So with the added salts the water was way to hard. So I did a 30% water change on the day and 10% water change the following days to graudually reduce the levels back down to normal.
During this time, a black widow tetra and a guppy also suddenly died. Another tetra was showing signs of infection due to stress so we went back to have the water retested... hardness was coming down, we were now half way back to normal. My husband bought some drops to put in the water to treat the infection, which was probably the result of stress.
The remaining fish seem to be recovering except for the infected tetra. They're swimming about normally and taking an interest in feeded. Fred was alive three hours ago when I checked him. But when i got up before he was gone. I checked in the clown loach's hidy hole to find she was dead too.
I couldn't bear to put them in the rubbish so I buried them both in the front garden. Fred's death has upset me to a rediculous level. i think because I have been trying so hard to save him. I'm dreading telling my daughter when she comes home from grandma's. I'm just hoping, that with exception of the sick tetra which i don't think will make it, that this tank is recovering and the rest of the small population will be okay.
The wierd thing is hoplos are very tolerant of all sorts of water conditions while bristlenose cats are very sensitive. Why did the hoplo get sick while the bristlenoses were still happily chomping away at the pebbles and glass doing their thing.
My fear is that it wasn't the water conditions that caused the problem but a disease that the bristlenoses introduced when I added them to the tank. In any case I know that it is something I did but I really wish i knew exactly what caused this.