diy concrete tiki

I wanted to hide my diy overflow pipes, and thought this might be neat. It is untested, so don't copy me unless you think it is fish safe. I live in the country and had these materials lying around, so I wont give a cost estimate.
First I cut some 6" diameter irrigation pipe to size and drew my designs. I then cut the holes out with a jigsaw and tied on some plastic animal fence with plastic poly baling twine. I think the fence material will help give the concrete something to hold onto. Then I mixed equal parts plastic cement and sand and added some powdered concrete color. Then I just sculpted the tiki man. So far I have only finished one, but I have 3 more ready to sculpt tommorrow. I will make sure to leach them until the ph stabilizes.

I know the concrete leaches lime for a long time. Other people make backgrounds, caves, and fake saltwater live rock out of concrete. I just thought I'd try a different form. And yes, the plastic pipe is safe for human use, so I'm pretty sure it will be fine for the fish. I'm not completely sure about the green fence material though. We had the concrete and coloring left over from my koi pond. There were'nt any problems for them, but I'm not sure it it will work in a 90 gallon aquarium.

Is it neccessary? If so, what kind do I get?

Well, nobody ever accused me of having good taste, but here are the tikis in my 90 gallon tank in my bedroom.