Mine just had their first boiled egg yolk meal. After that I changed their water 100% (I put them in some water from main tank) and then put them with that water from main tank in.
Before doing the moving, I mixed water from main tank with tub water so that they'd get used to it.
They're so active, I added more plants to their box. But I hope to find that tiny filter I had for my betta bowl long ago. Might be useful, even though ammonia doesn't build too easy here.
When I get my salary, I'll buy them a breeder net or build them a larger one and put them in main tank. That should take care of their water quality and they'll have like 25L all to themselves if I can get a big enough net.