Under some of mine, I just used thick (2"-3"), strong, styrofoam (which will adjust to inconsistencies in the floor on its own) and I used the kind, with foil type barrier to hold heat, directly under the sump/tank.
The same kind I tape to the back and sides of tanks. If gets soaked, cheap, and easy to replace.

below, another shot of float shut off valve sending water to the tank, there are other types available

here's a link, they can be life savers (time, floor, marriage)
Coralife Pure-Flo II RO Float Valve
The same kind I tape to the back and sides of tanks. If gets soaked, cheap, and easy to replace.

below, another shot of float shut off valve sending water to the tank, there are other types available

here's a link, they can be life savers (time, floor, marriage)
Coralife Pure-Flo II RO Float Valve