Hi there...Here is the best water change guide to answer your question. Also Prime will detox everything but only for 48 hours and then you need to add more, its 2 drops per gallon. You really dont add to do a water change everyday just read the guide and it will tell you what you need for your size tank and if it has a filter or not.
If anything, your water is probably too clean! I would suggest either following the guide in the sticky that Perseusmom linked, or getting and API master test kit and testing your water everyday for a little while to figure out what is the best schedule for you. :)
MuseBetta - Do you have a filter in the tank? Is there a reason that you want to do these daily 50% changes?
The recommendation for water changes on unfiltered 2.5 gal tanks is one 100% and one 50% water change per week. I don't see anything wrong with doing daily 50% and one 100% per week.... but that seems like a lot of extra work, unless you're trying to treat something (like fin rot, etc).....
Water can be too clean for a fish because good things grow in the water that help their immune system. So thats why it can stress the fish out. Also all the suttle changes in water chemistry can stress them out when water changes are done too often...
I think this depends on how the water is affecting your fish. If the fish's condition is severe, you may want to resort to frequent water changes. However, this can really shock the fish and deplete the healthy bacteria that break down tank waste. You can by neutralizing agents and a test kit to help you decide what to do. I would try the additives first before resorting to drastic water changes. If you decide it is what you need to to, make sure you are conditioning the water properly and making is warm enough for the fish.
Too clean? Or not fully cycled? I just moved my betta from a 1/2 gallon bowl into a filtered, heated 5 gallon aquarium for the FIRST time (2 hours ago). I'm not sure if he's just checking his new place out or not, but he's been swimming around like crazy. Is this normal? He does rest from time to time, but since I've never seen him so active, I am concerned that something might be wrong. I let it cycle for 24 hours, with Prime and StressZyme+ (recommended by Petsmart). I tested the water with a "test strip" which showed it was acidic, but all the other categories were within normal range. It doesn't measure ammonia, but I thought that with the new water, etc., it should be safe. I made sure the temperature was exactly the same as his bowl, as were the "test" results. The bowl has had daily water changes of 25 percent for the last 4 days. It's fun to see him active... but should I worry?