Tank question? Odd base?

[Hamamelis]

I bought a second hand tank a while back, and it's got a really odd base- there's some sort of hard black plastic lumpy stuff covering the bottom, it sounds hollow if you tap it, and it can't be easily removed.

I can't work out if I should be putting some polystyrene or something underneath- I presume the black stuff is designed to cope with any slight unevenness of the surface, but does that mean I shouldn't use anything else? I haven't had the nerve to fill it more than half way, which didn't really matter at first, as my lotls were only babies, but they're growing up fast. I'm moving house next week, so it seems like an obvious time to change it if it's not right.

Has anyone had a similar tank, that can set my mind at rest, or save me from imminent disaster?

PS, It's a 30" bow fronted tank, which is probably irrelevant.

Thanks!

 

[CJ1981]

Are you able to post a picture of the mystery substance?

I'm a bit of a worrier with my tanks and have all of them, even my fry tank on polystyrene or camping mats so while I would say yes to putting some under your tank I can't guarantee that others would agree with me!

 

[Hamamelis]

Unfortunately, I can't put up pictures, because the tank's in use (though it's not very full) and the stuff's only on the bottom.

I should have photographed it before I started using it really, but I wasn't thinking, and I was in a rush to get it set up.

I might just stick some under anyway, but I got a bit concerned because my other tank (a tropical fish tank) stated very clearly in the set up instructions that it should not have anything put under, and implied that doing so would increase the risk of the tank cracking, as it has a raised base...