Help with sick community tank with the surprise cory fry!

[palgrnstar]

1. Water parameters
a) Temerature range.
b) pH. =6.2 (though for months it was 7.0)
c) GH. =71.6 ppm
d) KH =71.6 ppm
e)Ammonia, Nitrate, Nitrite, leavels. A= 0 ppm Nitrate= 0 ppm Nitrite= 0 ppm
f) Water change frequency = ~30 % weekly, last changed yesterday


2. Tank set up
a) Size. = 20 gallon
b) Substrate. = petstore gravel
c) Filtration. = power filter
d) Furnishings. =two large "fake" rocks intended for aquarium use, with holes and caverns in them
e) Other tank mates. = 3 bronze cory adults, 2 bronze cory fry, 2 upside down catfish, 2 zebra danios, 2 neon tetras
f) How long has it been set-up? = Set up for 10 months

3. Symptoms / Problem description

Hello again, I corresponded with some of you about a week or so ago regarding the surprise bronze cory fry that I found in my community tank upon return from vacation. This tank began (9-10 months ago) with 5 zebra danio, 4 neon tetras, three bronze cories, and two upsidedown catfish. A few months ago, we didn't change the water in the tank frequently enough and the pH dropped substantially (from 7.0 to well below 6.0) (probably due to evaporation). I know it was low because it was off the chart on my pH test kit. I was able to begin bringing the ph back up with Proper pH 7.0, until I ran out. The LFStores around my area haven't gotten any in since then. Meanwhile, my non-catfish have been dying off. The two remaining tetras look pale, one of the two remaining zebra danios has >severe< dropsy (two others died after having dropsy), and the other one (just today!) started swimming upside down and spiralling around the tank like an acrobat!
Meanwhile all the catfish seem fine. I've tried to do as much amateur research as possible, and it seems likely that these fish all are having problems with internal bacterial infections. I've read about how when your pH drops like that, or other environmental stress occurs, bacteria normally in your tank can take the opportunity to infect your fish. This also makes sense to me because I've read that the catfish prefer a slightly lower pH than the other fish... so maybe the pH drop didn't bother them.
Anyway, it seems to me that I need to treat the tank, and the fish. I know that there may not be much hope left for the danios, but I am concerned about the others. First, should I be administering an antibiotic to the tank? I do not have a quarantine tank, nor can I afford one (both in terms of cost >and< space!) But will medicines designed to treat fish with bacterial infections hurt fish that do not have them, particularly scaleless fish like catfish (and >particularly< the cory fry!) Second, what more can I do to raise the pH successfully? For awhile, I was changing 30% if the water daily (our tap water has a pH of 7.0), but the pH in the tank now always "rebounds" to 6.2. I think the reason the Proper pH 7.0 worked well is because it had buffers in it. None of the other pH products I've found have buffers.
Additionally, should I be doing something to try and get rid of the bacteria in the tank? I siphon the gravel each time I change water, and my water tests indicate that the biological filter is well-established. I'd hate to kill everything off by using some chemical, but I just want to put this problem behind me. Any suggestions would be great. I'll keep checking this often, so if you have questions that I haven't answered, please ask and I'll write back as quick as I can.

Thanks SO much,
Sarah

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[Fiskars the Whiskers]

I was told to add a small shell or a little bit of coral to my tank to act as a buffer. My tap water is 6.8, but I had been lowering it and then found out I was not to do that! Since your water is already at 7.0, which is good, maybe the addition of the small shell or a little bit of coral will help buffer the water and bring it back up to 7.0? My pH would fluctuate and now that the shell has been in there the pH is always consistantly about 6.8-7.0. I'm sure someone else can give you a lot more information, but this might give you a little bit to go on?

[palgrnstar]

Fiskars,

Sounds interesting, though I don't even know where to go about getting a seashell or coral! I went to my LFS again last night, and they did have the Proper pH 7.0 in stock. I also bought Maracyn Two and I began treating the tank today.
Last night the extremely bloated danio died, but the cories seem to be fine. I still have the two fry in a guppy breeder at the top of the tank, and I'm wondering when I should release them to be with the parents. We were out of town for 2 weeks, and when we got back almost 2 weeks ago they were like half an inch long. Now they're nearly an inch long, but still small. They must be going nuts staying in this tiny breeder. The female cory just laid eggs again last night, and I'd like to put them in the breeder, but that would mean taking the fry out. I just don't know if they are old enough to do that. Additionally, I don't know if the Maracyn Two tablets on the bottom of the tank might hurt the fry somehow. Any thoughts? (This is directed to you and to anyone else who may be reading this!)

Thanks,
Sarah