While your tank is cycling, the ammount of water changes is determined in essence by the quality of your water. A fishy cycle can be a lot of work if you want to keep your fish healthy. If you have a test kit, test your water for ammonia and nitrites every day, and change your water...
Well, it really depends on a few factors such as
1) What kind of Bio Load do you have. Some fish are messier than others so the water would need to be changed more frequently and more of it.
If your tank is cycled then I would change 25 % of your water once a week and vaccum your gravel...
Has everything, that you have put in your tank, been prepared properly or has been approved for aquarium use. Sometimes, rocks and such that do not come from your LFS have stuff on them and can cause unwanted things to happen in your tank. I have read that as far as algae is concerned, I have...
Even though your platy is pregnant, you probably will not get any fry from it. If platy's are stressed they will abort their pregnancies and you wouldn't even know. In order for livebearers to give birth they need to have perfect water conditions. They will mate in less than perfect...