The fry in the first week or two are pretty weak swimmers, so the tank should not have deep water or any significant currents. But the water needs to be of high quality, with no scum on the surface, and plenty of oxygen. The fry gulp air in the first week or so to fill their swimbladders: if there is scum on the surface that prevents this, the fish will never develop proper buoyancy control, if they even survive.
I started with air-driven sponge filters and air stones in the tanks. I changed huge amounts of water through a complex dripper system. If I were doing it with buckets (which I ended up doing for additional later tanks), I would try to change 10 to 20 percent per day at least. More if the water seems foul. Use a siphon to get the poop and uneaten food off the bottom. As the fish got bigger, I put power filters on the tanks (mostly Aquaclear 200s). Initially one per tank, then later two so I could alternate cleanings. The filter intake sieves need to be covered with something to keep the fry out. I used plastic pot scrubbers.
By the time the fry were a few months old, it was clear to me that their growth was limited by the tank systems I had. Without aeration, they would be piping at the surface of the aquarium. The filters would become clogged every 3 or 4 days and would require cleaning. Essentially this means that the amount I could feed the babies was limited not by what they could eat but by what the tanks and filters could handle. This is the very definition of overcrowding! Unfortunately I dont think there is any way to avoid some degree of this, but the risks are reduced growth, susceptibility to illness, and potential losses during any power outages. Oh, and a few hours per day of tank cleaning.
For perspective, I moved a few fish to quarters where they had 5 gallons per fish when they were about one inch long. These fish quickly accelerated in growth past their siblings in more crowded tanks. A couple of months later they were 3 to 4 times the size of their more cramped counterparts.