Ya, I have a friend that breeds them, and only the breeding adults are in RO.

Ha ha
 

Beta typically live in dirtier water. Labyrinth fish haave two sets of lungs that allow them to breath our air when needed. Thats the reason they gulp at the surface and that some people feel its ok to let it live in a tea cup.

Some people add "black water" additive for betas as well.

So... sure RO is probably ok... but overkill for a beta. Save your money.

When I lived in Thailand I used to see Bettas living in their ponds, they really don't move around that much 'cause movement alerts predators like the nasty turtles that love to eat them. Being in a smaller tank/bowl just isn't ideal. I even keep several Bettas (1 male : 8 females) in a 6 ft assorted tank...they really just sit on plants near the surface and chill. ;)

Why not use a 50/50 mix if you have some concerns?

Well, had the RO/DI here so figured if it was ok to use it...
Was hoping it would eleiminate any unwanted alge etc into the tank. I always here of so many fw folks having algae issues and alot of them seem to use tap water.
The point being, now that the tank insn't here in my place anymore, the cleaner it can stay the better as I don't see my sister doing a lot to take care of it

You could always add a couple snails to eat the algae.

Think we';ll just run with the RODI water for now and see how it goes.
Guy at teh LFS says that is what keep their FW fish in so that is what he has been in all along

It will take a while to kill him anyway. :neutral:

At the very least, you could do him a favor and use Aragonite sand if you're going to stick with the RO/DI so there is at least some amount of pH buffering capacity.

Fyi, in most cases you can use the waste line of the RO/DI unit for most freshwater fish.