Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Expanding foam filler is sold in most hardware stores. There is also a black material sold for pond use. All are fish-safe when they are dry. They can be shaped a little bit when they are wet, but not very much. They can be carved when they are dry.
I have used the hardware stuff in a tank to make a retaining wall, rocks and foam.
I have used the pond stuff both in ponds and tanks. Wear gloves. It sticks for a LONG time!
Use the whole can. (Line up several projects at once). The best way to store a partially used can is not economical unless you are a pond installation contractor.