My gramps pond is leaking water, and i suggested fixing it, now he tells me i can use it for my own fish if i fix it. The pond is made of those small 8"x4" bricks cemented together and the inside was concrete washed before. Is there some kind of sealant i can use to just roll onto the concrete to seal the leaks? Thanks for any info. Pond is about 5'x5'x18"