Sudden die-off, please help

I recently set up a second tank to breed feeder guppies. Everything was going great and I even got some fry that first day, but after a few days I noticed that the plastic plants I had borrowed from my main tank were not enough cover for the fry and they were all getting eaten. I built a mesh tank divider that the fry could get through and set it up on an angle. The idea was that the newborn fry would essentially fall out of the mesh and then stay on the "safe" side. I set this up last night. I don't know if it would have worked because today when I got home from work, all but two of the fry were dead. I quickly removed the mesh and did a 50% water change (replacing with water from my main tank, which is cycled), but one adult and one juvenile died while I was doing it.

My question is not "what did I do wrong." I did many things wrong---many stupid things. Chief among them was failing to research the mesh material before putting it in the tank. And I feel terrible for causing unnecessary deaths. My question now is, what more can I do for the surviving fish? I'm sure the fry won't make it, but I hope the remaining adults can pull through. They seem listless and their swimming looks a bit uncoordinated, but I can't see anything else physically wrong. Besides continued water changes, what can I do?

Thank you for reading. Any help would be greatly appreciated. I don't want them to suffer.

I'm afraid all I can tell you is that the ammonia level is zero and the pH is about 7.5, slightly lower than it was two days ago (about 8). I don't have nitrite or nitrate tests, but believe me I'll be buying some tomorrow. The tank doesn't have any algae growth that I can see (my only indicator of nitrates), but it does have one rather sad (but recovering) java fern, which could be soaking up all the nitrates. I've done water changes about every other day and I've been using water from my main tank, which (I believe) is cycled. The guppies in there are certainly happy enough, but they weren't purchased at the same time or from the same store so that might not mean anything. I just did some more reading about cycling with bacteria from an established tank, though, and it sounds like my water changes wouldn't have made any difference because the bacteria aren't free floating in the water. Should I grab some of the filter media from the main tank and put it in the breeder tank? I don't have the same kind of filter in each, so I would just have to literally toss it in. Would that do any good?

I had one more death this morning---one of the adult females. I found one of the two remaining fry still swimming. Couldn't find the other one. The remaining two adult females and one juvenile male seemed okay, but I'm worried about what I'll find when I get home.

DrVader - My ammonia and pH tests are liquid tests. I'll try to pick up liquid tests for nitrite and nitrate after work, but if my local store (Agway, not a real fish store) doesn't have them I may not be able to get to my LFS before the weekend.

Zigman - I don't know what the mesh is usually used for. I got it from my hardware store. I believe the tag said it was PET plastic, which is used for food containers, but maybe there was something left on it from manufacturing that didn't come off when I washed it?

Hooked Newbie - Thanks for the tip, I'll stick with fresh dechlorinated water from now on.

KarlTh - Clearly, I didn't understand this whole cycling thing nearly as well as I thought I did. Thank you very much for clarifying about the water. The fry were on the filter side---not my first choice, but it's a small tank (10g) and I couldn't get the mesh and the filter to hang on the same side of the tank. I'll move some filter media from the main tank and keep my fingers crossed.

sinner - I'm hoping to get some java moss and java fern very soon. Trying to find it locally and not having much luck. If I have to order online, what's the best source? I keep coming back to aquariumplants.com, but I'd appreciate any tips. I do want to get a significant amount of them for the breeder tank so that I don't have to wait for them to grow in, so price is definitely a factor. It's also very very cold here in NH so I'm concerned about shipping.

Thank you all so much for your help and support.