I am going to build a indoor pond (8'X4'X18") using a support frame made up of 2X4's, I plan on having the base of the pond on the floor and using additional support of concrete slate pavers as additional support (with the 2X4's) for the areas of different heights. While researching this I have seen some build indoor ponds walls using plywood, others have used sheetrock... is there a better option? Secondly I am currently using rocks for my substrate for my tank. I read it is better for turtles to use pool sand/gravel, however I am unsure how safe that option is for my filter. I have bought a Tetrapond clearchoice PF-3 for this pond and am unsure if the sand will damage this biofilter. Lastly I had also seen how I may be able to avoid having to heat the pond water because I keep the room with the pond at a 70-75 minimum temperature.... Is this accurate? I am hoping to get some thoughts, ideas, suggestions and tips to avoid as many possible problems along the way as I can. Please and Thank You!