bettas die one by one

Question
ve a sixteen gallon tank which started off having 5 female bettas in it.  They were doing fine and not abusing eachother besides a nip here and there.  a few days ago one died and i thought maybe it was a weak one and it happens.  Today another one died in the morninig.  I came home tonight and found one hiding and not acting like her normal self.  I have no clue what is happening.  I checked all the water levels and they all seem fine except that the ph is around 7.8.  Is that too high , could it be killing the females?  I also have three good size rocks in the tank tank that i got from a fish store.  Maybe the rocks are releasing harmful chemicals into the water?  I do freguent water changes and check the levels often.  I hate seeing the fish die one by one.  I feel helpless and wish i could do something about it.  thanks
Fred

Answer
Hi Fred,
I'm sorry to hear about that. Even if the water tests out good (ammonia-0 Nitrite-0 Nitrate-20 or less) I'd still do a 30% or so water change everyday or so just in case. Its better as a precaution.

The pH is not an issue in general with bettas, and the current level is just fine for them.

The rocks generally should be safe if you bought them from the petstore from the fish department but you never know. Are they natural rocks or something colorful and artificial? Might be good to remove them if they are the latter.

Best of luck and I hope things improve!
Karen~