27 days post spawn, or post hatch, or post release?
Until they have swim bladders and look like fully formed fish (not much of a visible yolk sack bulge), they won't be able to eat. When they can, I'd estimate around day 24-26 post spawn and within a day or two of being released from the male's mouth under normal circumstances they'll be able to eat, and I'd recommend copepods if possible. They shouldn't be that picky when it comes to live, rotifers may be accepted but are worse nutritionally and are really small for their size, so I'd probably hatch out some atemia nauplii as supplemental food if your stock of copepods isn't huge (they can eat a lot, and they benefit from fairly heavy feedings, regularly.) The feeding response should be quick - if you have them in a relatively confined space and offer up some food to fry that can eat, they should start going after it well under a minute from it being offered.
Starting from around a week from that point, I'd offer prepared foods, and my experience has the best acceptance with frozen. Something like calanus, cyclops, or frozen brine shrimp nauplii should work fine, but I've offered mine the fines of mysis shrimp and they will take to that as well. In my experience small pellets take a little more time for them to figure out (less time drifiting in the water column, I think), but a smaller pellet like TDO B1 or so could also be used around this time. Gradually larger foods can be used in the next couple of months, but at least with mine, once you get to the XSmall TDO size or so, they just don't want to eat bigger pellets, even though juveniles will gladly shove a mysis their entire body length into their mouth when given the opportunity.
