Freshwater tank out of control?

Question
Hi :) Hoping you can answer my questions!

My husband and I recently bought a 10 gallon tank, as our Betta appeared sick or depressed we decided to try everything in the book to cheer the little guy up, added two Zebra Danios. Unfortunately nothing worked, not even the several meds we bought, he passed away soon after we purchased the tank. So we decided to go the distance and start our own aquarium, adding two Corys (spotted, I believe), and two Silver-tipped Tetras.

Doing our best to keep our fishies healthy, we changed the water daily to adjust PH/ammonia levels, and they've been doing just fine; eating, playing, swimming. Finally coming around to the point :) recently I've noticed our little Corys acting strange, hiding in their cave for large amounts of time and not really been eating much, also swimming frantically away from us(they never used to). Tonight, I did a partial water change and they seem to be cheering up, wondering what could have or be the issue with that?

Onto my next question.. I noticed one of the Zebras was incredibly fat, I -think- it's the female, it's lighter in color and has less stripes. Seems to be slightly red around it's lower regions, literally looks like it's about to burst. Should I assume it's pregnant? If so, what should I do? I don't want the babies eaten, and the Silver-tipped Tetras seem to be interested in chasing her and picking at her, it even looks like her tail is deterriorating from it.

Any idea what's going on here? Any info would be great. :)

Thanks a bunch, :)
~Tina

Answer
When your ammonia levels (which should be at zero for an established tank) go down, your nitrites and nitrates will be going up.  These chemicals will also make you fish sick.  The levels of all three should be zero in a healthy tank.  You cories are probably reacting to the water paremeters.  Here's a site on how the bacteria in your tank breaks down the ammonia.
http://www.cichlid-forum.com/articles/fishless_cycling.php

I don't think  your zebra danio is pregnant, they are egg layers and don't generally get any bigger.  It sounds to me like it is sick, especially since the other fish are picking on it.  I would have to suspect bloat from your description.  You can try treating with a product called "Clout", but it is more expensive then your fish, so it is up to you.  The chances of successfully treating a fish that has already started to swell is low.  The best thing is to help make a healthy environment for your remaining fish.  Get your other levels tested and do some 25% water changes each day until the levels are at zero.  Then you must do a 10-25% water change every week and keep your gravel clean.  Make sure you are providing adequate filtration, heat and aeration.  I hope this information helps you.