Okay, this is what i did with my tadpoles, I only had seventeen so it will be a little different because you have so many. I only changed the water when it was dirty, so if that meant 3 days or a week I just went with my instinct. If I remember correctly I would say I changed 3/4 of the water about once a week. My tadpoles were in a 20 gallon long with a third of that gravel as I mentioned in an earlier post. My suggestion is to put the gravel in there now, it won't hurt doing it before they get there legs and they like to hang out on and around the gravel. I have read that changing their water too often can be harmful, and not enough can cause a build up of an anti growth hormone (I do not know if this is true, it's just something I read when I was researching how to take care of Gray's). I therefor changed the water not too much and not too little. I used a clean styrofoam cup to take the water out of the tank and NEVER took the tadpoles out, as this would have caused them way more stress. Just change the water with them in there. Take out about 3/4 of the water, that should leave enough room for them to swim while your filling it back up. I used a small fish net to get all the poo from the bottom, this really helped. I think you can pick one up for a few dollars at any pet store or wal-mart. This is where the gravel comes in handy, because instead of pouring the water directly in the tank, you can pour it on the side of the tank (slowly) and it goes between the glass and the gravel. It won't disturb the tads this way. I have not had any of my tads nip at each other but the ones that I saw outside in my friends pond would occasionally dine on one of their siblings (it looked like it was dead or almost dead, so I can't say if they killed it or were just being opportunistic cannibals). As far as your concern about them drowning, put the gravel in like I mentioned before and you will not have to worry. Mine never came out of the water before all four legs were present. After you see all their legs, keep an eye on them. After they popped all four legs out, they would sit on top of the gravel just barely below the water and poke their heads out sometimes. This way I knew that even if they stayed in the water, they had the option of poking their heads out and breathing if they wanted to. Sometimes, even when I thought that they were going to come out I would check back an hour later and they had went to the bottom of the tank again----I thought "OH NO!! They are going to drown!" But they never did because I had gravel in there. They eventually climbed back up when they were ready and climbed up the glass. I never had any of mine splash around like you mentioned, so I can't say if that is good or bad. I don't know what the correct size of tank is for keeping tadpoles but mine had more than enough room in my twenty gallon. I wouldn't worry about covering the sides. Mine were not covered and the cats sat right next to the tank and pawed at the glass. Didn't seem to bother them at all. Oh, by the way, I had the same problem with my two little runts too. They were only half the size of the others and only lived for about a week in my tank. I don't know why they died but it was very sad. I thought they looked healthy but they just never seemed to grow and always stayed around the bottom of the tank. I hope this helped. I know how worried and frustrated a person can get when your trying to help animals that you have limited knowledge of. Ask me to take care of a dog, cat, or horse and I can do it blindfolded. Ask me to take care of a frog two months ago and I looked like this