Here is the link to a company that sells the perfect foam for use under a tank. This is the same stuff I purchased at Target; only they offer thicker versions.
Exervo Interlocking Mat Set - 24 sq. ft, 3/4" thick set
I would use the 3/4" exercise mat for tanks up to 125G.
Exervo Interlocking Mat Set - 24 sq. ft, 3/4" thick set
I would use the 3/4" exercise mat for tanks up to 125G.