Reversed mounting ring certainly wouldn't be my choice when it comes to aquarium photography. It provides great magnification (for it's price) but very very poor working distance.
The 28-90 should work fine with it. The zoom will give you a range of magnification with the ring.
You can get them off Ebay for cheap and if you ship by express, you should get it in a few days. I purchase on Ebay frequently and those are often big ticket items. I am careful, but I never had an issue.