Why the hell on earth would you use, what looks like to me, a glass coffee table as a stand for a very heavy fish tank. That stand looks terrible, plus it's now broke, and you still insist on bodging it. Throwing some ply on top of that broken top isn't going to make that stand any more robust. Build a new stand for peace of mind. As mentioned, if cost is an issue for you, use cinder blocks with ply on top. That's about as cheap and sturdy as a stand can get.