Emergency!!!! Tank Crashed!!!

[trashion]

i came home from work today and noticed my tank was cloudy. i turned the light on and OH MY GOD.

it's RIDICULOUSLY cloudy, all four of my cardinals are dead(!!!). my skirts were gasping at the surface along with my otos. and there is this WHITE CRAP floating around!!! it's white, cottony-ish threads, it's stuck to my plants and it's in my filter.

tank specs:
15 gallon tank, stocked with 5 skirt tetras, 4 cardinal tetras, 3 otos. temp 81 degrees, w/ aquaclear 30 filter. planted moderately with eco-complete substrate, flourish dosed biweekly.

i immediately tested the levels:
ammonia: 0
nitrite: 0.05
nitrate: i don't have a nitrate test
pH: 7.2 [.6 lower than normal]
CO2 concentration: 6 ppm

i immediately put in an airstone, and then carefully netted out all of my skirts and ottos and put them into my 6 gallon eclipse betta tank. my betta is dwelling in his old 2.5 gallon for the time being. i moved all of the plants/rocks/driftwood into the 2.5 and the 6 as well.

possible culprit: i double-dosed CO2 today. i'm trying to use up the tablets for my jungle CO2 fizz factory, and since i got no effect with 1.5 tablets, i used 3.

i took the filter off the back and it's absolutely filled with greenish yellowish slime. i had to rinse the tube, the media, everything it was disgusting. I REALIZE i should not have rinsed the media, because i probably destroyed the beneficial bacteria, but i was freaking out.

WHAT HAPPENED? what is this white crap? what can i do to save my fish? they seem to be okay, albeit a bit crowded in the 6 gallon [it's cycled, don't worry] how can i get rid of the slime floating in the water? HELP!

[emc7]

I'm guessing the white crap is "dead fish fungus". Remove the corpses and siphon out the residue and it should go away. Was the filter running when you got home? Its possible the slime (some kind of algae, most likely) clogged the filter media enough to block the flow.

WHAT HAPPENED?

I can only speculate but I'd guess that you hit the nail on the head with CRASH. The CO2 cause a sudden drop in pH that either did in some fish or wiped out a lot of algae. The other possible root cause is your filter not working because it became clogged, that combined with the CO2 would mean the fish would have trouble getting enough O2 and some could have died of it. Everything else is the result of having a lot of dead, rotting stuff relative to the size of the tank. You must have had an ammonia spike since you had a nitrite spike when you tested. The cloudiness is bacteria multiplying to take advantage of the extra ammonia and nitrite to eat, the white string are fungus feasting on dead fish, and the slime is either a rapidly growing algae or dying algae that sloughed off the surfaces it was clinging to. The gasping for breath was the result of ammonia/nitrite poisoning which may have killed more fish than the initial pH drop.

how can i get rid of the slime floating in the water

If its floating on the surface skim it off or soak it with a paper towel, or better yet, drain the tank and rinse the the substrate.

what can i do to save my fish?

You've done it, the best cure for poisoning is clean water. You may have a few more loses but if your fish are acting fine, most of them will recover. Consider buffering your water with baking soda to prevent this from recurring. Always watch pH and hardness when adding CO2. I really think that if you refill the tank with clean water, use Prime or Amquel+ to lessen the affect of ammonia, your tank will have a little mini-cycle and bounce back to normal quickly. You may wish to leave some of the fish in the little tank for a while, but don't leave all of them in there or the overcrowding could hurt them. Watch the ammonia and nitrite in both tanks for awhile.

I always suspect water quality over disease when several fish die at the same time with no prior symptom. However, if you find white stuff attached to still living fish it could be columnaris which kills rapidly. If you see this or other symptoms, start reading the disease sites. Otherwise, it was "just" water quality and your tank will be "fixed" by clean water and a little time. Even rinsing in warm tap water will not totally destroy the filter's colony, so it should bounce back and you won't have to "re-cycle" the tank from scratch. And if slime was restricting the water flow it needed to be done.
Things like this are why I tell people to get a bigger tanks and/or fewer fish than you want. It gives you some insurance. But I don't think your 15 was overstocked, its just that smaller tanks are much more prone to pH swings (and hardness swing and temperature swings). Please think twice before trying CO2 in the 6.

[Obsidian]

I would do a water change and do NOT scrub your decorations or substrate since you did rinse out the filter. You can rinse them off and by hand remove any debris in the OLD tank water from the water change. Personally I would do this and let it run overnight and then do another W/C tomorrow and rinse things off in that water again. If the water does not start to clear I would be more concerned in terms of long term problems with the tank. If it starts to clear up then I would guess it has something to do with the tabs you used. Other than that I have no idea what would have caused this though I am still too new myself.

[JustOneMore20]

pH swings from co2 (carbonic acid) don't kill fish..........and 6ppm of CO2 doesn't either.

Good info above though and I think you've done what you can do. I am not sure what those tabs are made of but it seems like they are the culprit. I don't think the pH drop or the 6ppm (if thats accurate) of co2 did it though.

[emc7]

Blaming the tabs is just a guess. Hard to know what happened first. In a large tank, one fish dies and it just becomes fish food, in a small tank it sets off a chain reaction. I would also suggest adding a second filter, even just a sponge on a air pump, as insurance against mechanical failure. I've seen pH shock kill angels, but only when the pH went up suddenly (like me moving it to a another tank without testing it first). Riverine fish are usually pretty tolerant of pH swings. Cardinals should like lower pH, but you can't tell how suddenly it dropped or if it dropped further and then came back up. Its really only one possibility. How long have you had the cardinals?

[TheOldSalt]

Hmmm... all this happened in one day?

[trashion]

yes. i fed my fish before i left for work, the tank was perfect except for a bit of algae. i came home and it was cloudy and 4 fish were dead.