ChuckG,
What your looking for are called insert glides. I know took me forever to find them for a stand i built for my brother. So, I think this in an example of what your looking for.
Plastic Furniture Components
About 3/4 of the page down.
The one you want are the ajustable ones. Also you will have to get the insert gromets. Check out the PDF links they give a little more detail on them.
Edit: Oh and one more thing i forgot to add, the weight allowance on them is "Each". So you can add the four legs up to hold your weight of the tank. The industry also does that with casters.