KrazyKat said:
Dont mean to repeat what Roan said but aslong as you already did some waterchanges - and the temperature of the water you use is the same as in the tank (and its treated) then you wont do him any harm.
Every now and then i do 2x50% one after another - because when i really vac my gravel the 50% are gone and i am not done yet - so i fill back up and finish my gravel vac.
If anything - it really seems to excite my betta. Maybe its because of all the clean water - or the fact that there is always a bit or tank rearranging going on which means he can "explore" whats different.
It doesnt hurt though....