Is there any algae anywhere within the sump chamber? Most whiptail fry I've raised seem to spend most of their time nibbling on biofilm of some sort. They may also benefit from frozen daphnia or decapsulated brine shrimp eggs. I've also had reasonably good luck tossing in some established filter media with small amounts of mulm in a larger established tank with fry (The mulm can and will shift water parameters for the worse in smaller tanks with inadequate filtration), but this doesn't always seem to be that useful as the last couple batches of BN fry I raised completely ignored it. If possible, partially degrading leaf litter from mature tanks seem to harbor microorganisms or biofilm most BN pleco fry seem to like; I would assume whiptails would be the same. Blanched courgette might be beneficial to add; it would be rather surprising if none of the fry even attempt to eat it. If you can find any, Repashy Soilent Green is a good "paste" of sorts (comes in a dry powder) when prepared right and is my personal favorite staple for rearing pleco fry. If unavailable, the algae pellets that you have would probably be a reasonable substitute.
The biggest concern with fry is sanitation; fungus and bacterial growth, if left unattended, can wipe out an entire group of freshly-hatched fry (although this generally isn't a problem in more mature setups). All food uneaten after 12 or so hours should be removed and replaced, and any unhatched eggs with fungus should be disposed of as soon as possible.
