re-using discus tank water for bettas

[Etek]

So I've been keeping bettas off and on for the past several years. I usually have good luck with them using no additives to my water and not aging it either. But in the past several months..I've had a rash of unexpected deaths. So I experimented refilling the betta tanks with the discus tank water right before I do a water change. Its been 2 months now and the bettas are thriving and I don't feel bad just wasting water.