At that reading you shouldn't have to flush the pots, especially if you're top watering and allowing water to run through on each watering. That will help prevent anything from building up. Wish I could use tap water!
I've been keeping Nothobranchius palmqvisti for quite a few years now. At first I kept every fish that hatched (moving the non-breeders to my planted tank) but since they mature so quickly they really do get aged and sickly looking fairly quickly. I end up feeling bad for keeping them alive than nature has taught them to. So basically I do what most annual killifish people do, keep one male and 3-4 females for breeding and cull off the others. And once I get a few bags of eggs, cull off those ones.
I've always used tap water for them, I tried RO water at one point because someone mentioned them liking that... and the freshly hatched fry died off. I'd assume PH swing or something but really don't know. Went right back to using treated tap water. I have a few topics on TF regarding annuals....
I tried rachovii Beira 98 once, but didn't get any to hatch, I think the guy sent me some cultures of other live food too saying their mouths as babies are really small and couldn't start off with brine shrimp like other annuals.
I kept a couple cycles of Nematolebias papilliferus too, but gave away my eggs, man they eat a lot!!