For some stange reasons most of my goldfish are dying......... I don't know why. I recently lost a koi, and 2 comets. So now I am down to 1 rybunkin goldfish, 1 shubunkin goldfish, and 4 fancy goldfish who all look very healthy. The ones that died were healthy but I don't know why and how they died. Tankmates are a rybunkin goldfish, shubunkin goldfish, 4 fancy goldfish, and a pleco and 2 cats. I just hope that the rest of the goldies survive so they can go in the pond. I am so sad right now..... I am going to have to pick up another koi and more comets this week. I will be trading some fancies this week though.
Alex

Sounds like too many fish for a 20gal. What are your ammonia, nitrite and nitrate readings? My guess is your parameters are out of whack because of all that bio-load. What kind of pleco and cats?

Checking all the parameters and a water change would be a good start.

I'm not sure if it's a bio-load issue. I've pulled golides from pure filthy mud stagnant crap water and they've been fine. They're usually tough little buggers.

I have had something odd happened to me with one of the comet tubs. I got in a big batch of feeder comets, (3-4"ers) and had them in the usual quarantine tub. They all look healthy and fine and then after a week, out of no where a good half of them droped, with no apparent symptoms. I finally did a necropsy on one that had just died, and after doing a scrape I had found they had Costia. I threw in a heater, cranked it to 82 and left them. After a week, the dying stopped, and things seemed to go back to normal.

I'm not sure if it's a bio-load issue either because I haven't seen any testing done yet. But 9 goldfish, a pleco (of some sort) and 2 cats (of some sort) in a 20gal tells me he is well on his way to a bio-load issue. An ammonia or nitrite spike from adding fish too quickly could also be an issue.

With that bioload on a 20 long, ya need to be doing water changes every other day at a minimum, BSF, even if it is a short term arrangement. I agree with saint_felony that goldfish are sturdy fish, but only to a point.

How much aereation and filtration ya got in the tank? Are your fish showing any symptoms? Gasping at the surface? Excessively flared gills? Are the gills normal coloration? How are their appetites? When was your last water change?

Check the nitrites and nitrates ASAP. Don't feed for 48 hours and throw an airstone in there. Make sure ya got good aereation. c_scherer123's suggestion of 40% w/c every other day is a must with that bioload. Nitrosomonas are most efficient at a pH of 7.9-8.2 and Nitrobacter at 7.5-7.8, IIRC, so raise the pH slowly over the next couple of days.

WYite

Ok guys heres an update... goldfish seem to be doing ok. I am still montioring though. Will be doing a big water change tommrow, and checking ph. There is only 2 plecos and a pim blochii in there now. I might move them to a 40 breeder intil spring.

Sorry about your comet and koi
It seems like an ammonia spike to me. Way to many fish for that tank IMHO. Goldfish need about 10g per fish at halfway grown-full grown. Goldfish are ridiculously dirty because they have no stomachs so waste builds up much faster, and plecos are notoriously dirty as well. I would move them to the 40g like you were thinking and water changes frequently. I hope they’ll stay ok!

that's good to hear! I bet the pond that your planning to put them in will be awesome.
must post pics in spring!

Glad to hear your fish are doing better BSF.