Fry are not eating BBS / Artemia

[TCMontium]

My Apistogramma fry are now 1 month old and they grew 1 cm long and bigger (3/8-1/2 inch, their size varies). Until now they mostly ate BBS / Artemia and rarely dry baby flakes. Now, all of a sudden they started to eat and just spit out the BBS. For last 2-3 days there were some fry that did not have a "full belly" after I gave them BBS and everyday the number of such fry increases. Today I saw only 2 out of 18 fry ACTUALLY eating BBS without spitting them out and those fry have "full bellies" as they allways did after feeding. Others seem to have nothing in their stomach.
I gave some No Planaria into the tank yesterday because of hydra problem, but I changed the water %50 percent after 16 hours. I don't think No Planaria has something to do with this problem. Some of the fry didn't eat BBS before I gave No Planaria anyway.

Maybe hydra stings made them not like BBS? Maybe they are bored of the taste of BBS? Maybe they like dry baby food more? Or is there definitely something wrong with my water (I change the water 70% twice every week with RO water)? What should I do before they starve to death?

 

[gerald]

Are these live fresh-hatched BBS? If not live, maybe the food has spoiled? It's also possible that Hydra stings or the drug for treating Hydra has suppressed their appetites. If they wont eat live BBS then I'd be surprised if they will eat anything else.

 

[TCMontium]

BBS are freshly hatched live ones.
As I said, some fry weren't eating even before I used a drug to kill the hydra. The chance of the drug being a problem is the slimmest.
Now, after 7-10 days of the beginning of the problem, most or all of the fry seem to be eating BBS again. Some of them are smaller and skinnier then others the and they eat less, so I can't really tell if they do eat BBS as the others normally do OR they eat-spit BBS and rarely swallow some.
The timing of them rejecting and then returning to BBS is conveniently in syncronization with the hydra population jumping up and then hitting the buttom. The number of the rejecting fry changed daily and slowly too, so that makes it even more likely for the hydra-sting scenario.

Maybe I am wrong and there is a completely different situation, but to my extent of knowledge hydra-sting problem seems to be the most plausible explanation.
I will write if I observe anything of value about the situation.