I built a small pond last year but forgot to add waterproofing to the cement. No harm done, or so I thought, G4 Pond sealant to the rescue. Having painted it around the whole pond (two coats) I filled it up and the water leaked out within about 10 minutes. The bottom inch or so of water doesn't soak away, that suggests to me that there's a hole somewhere roughly where the water level is. Having already mopped up the water and stuck another coat over the top that made no difference I thought that I could fill it with water and add a blob of colour that would show me where it's leaking out. I have, however, no idea what I could use that won't just mix in with the water. Does anyone have any ideas? Best I could come up with was food colouring (but that will mix) or oil (but that floats so probably won't work).