Mysterious Multiple Fish Deaths

[fishboytoo]

I had a mysterious catastrophe in one of my tanks today which so far has left 6 out of ten rasboras dead and some pepper corys looking like they might join them.

The tank is 18 gallons, moderately planted, with a black sand substrate(3-4cm depth), some drift wood and river stones for decorations. Occupants were 10 rasboras, 4 paleatus cories, amano shrimp, red nose shrimp and horn snail. The contents were transferred from a 10 gal to the 18 over a month ago, but had been running for 4 years previously.

I did some regular maintenance today 50% water change, replaced filter media, cleaned glass, vacuumed sand. I also added some bacterial culture and plant fertilizer(Kent Plant-gro) after the water change.

Everything was fine and then about 3-4 hours later there were four dead rasboras(hengeli and heteromorpha) in the tank and the rest looked in distress also. They were at the top not moving much kind of rolling over side ways or nose up/down. Also three of my four pepper cories looked poor with two staying at the top on their sides.

I took a water sample then immediately added two large doses of two different water conditioners. And yes, I did add it to the replacement water during the change. I then checked the water stats. Ammonia -0, Nitrite- 0, chlorine- 0. The fish looked as though they were suffering from poisoning or oxygen depravation. I added and airstone to the tank. Two more of the poorer looking rasboras have since died despite being transfered to another tank. The last four are looking better now and only the one cory is still at the top, and he seems a bit more lively now also. They are not 100%, but I believe they will survive.The invertibrates were unaffected.

I can't quite make sense of it all. I changed the water in four other tanks today, following pretty well the same procedure and they are fine. The only explanation that I can come up with is that while vacuuming the sand and generally mucking around in the bottom, I released a toxic gass pocket that caused the problem. Either the water conditioner neutralized the compound or the airstone drove off the dissolved gass. I forgot to mention temp. remained constant.

Anybody have any ideas as to the cause of this massacre? I don't want to replace these fish until I can ensure it won't happen again. Not to mention securing the safety of the remaining inhabitants. Any suggestions are welcomed.

 

[WILDER]

Whats that bacterial culture you have added to the tank, something you have added they didn't like, plus you should of increased aeration when added the chems.

 

[fishboytoo]

Thanks for the reply Wilder.

The bacterial culture is called Cycle by Nutrafin and is supposed to contain nitrifiers to help with the biological filter. I have used both cycle and the Plant-gro, which is a weak fertilizer with trace elements, off and on for years and have not had any problems with them before.

Why would they all of a sudden be harmful to the fish? The other tanks which I added these same chemicals to showed no signs of a problem.

BTW, this morning the rasboras are swimming together and the cories seem their usual selves, scurrying about on the bottom. Whatever it was, it seems to have been nullified. It is still troubling to me that there is no obvious cause.

 

[WILDER]

You only add cycle when cycling a tank, when you have finished using a med, or when you have rinsed a sponge in tank water, it sounds like something in the water change to me, anybody been using your bucket for household cleaning.
Could anything of gotten into the water during the water change.

 

[fishboytoo]

I added the cycle this time because of a new bio-max insert, ceramic cylinders, which I introduced. I was hoping to help "seed" the insert as it were. I also find the water looks polished after I use the cycle, I don't know why though. You wouldn't expect it to make any difference but it seems to me that it does.

In regards to the bucket, it is exclusively for my fish and I had used it to change water in another tank before this one. I am thinking it may have been the filter which could have been contaminated with chemicals. I left it by the sink while my better half was cleaning in the area. She said she doesn't think that she got anything on it, but perhaps there was some cross-contamination from us both using the sink around the same time. It seems a perfectly reasonable explanation. I wouldn't have thought it probable though, or I'd have avoided it in the first place. However, as Sherlock Holmes put it "We must fall back upon the old axiom that when all other contingencies fail, whatever remains, however improbable, must be the truth."
I suspect this is a one time, freak occurance and I can rest easier now.

Thank you for your advice Watson... um Wilder