I think the cheapest way to heat / filter your fry is like i have.
I found a 4ft by 3ft tray which would hold water, filled it half full of water, dropped a heater in it.
thn sat my containers wth the fry in, into the tray. eventually the heated water warms up the water in your fry's containers.
The containers i have are plastic, so what iv thought about doing is putting lots of holes in the side ( not big enought for the fry to get trought) and then do as i have with the heater and drop a filter into the tray.
Iv got 25 in the tray, then iv made my own wooden frame, 4ft by 3ft, lined it with a large plastic sheet, and it does the exact same job.
1 heater, 21 fish...