Ye ole google says that koi can go up to 90, but seems like they really don't like it once it gets over 85. They prefer it in the 50-70's and don't fare as well once it's into the 80's on the regular. So I would guess according to the quick google look that 88-90 is probably too warm for the fry to be happy with for very long.

I ended up putting the koi fry into my 75 gallon tank (75 degrees). I separated the larger from the smaller koi. I started feeding them once a day till my tank ammonia and nitrite went to 0 then I went to feeding them 2 and then 3 times a day with koi flake food. My nitrate has been about 30 when I do a 30% water change every 3-4 days. I have a Marineland C360, 2 sponge filters, TOHIDAQU sponge filter and a small air stone.

Why an I losing 10 of the smaller koi a day?