Wow, that's a lot of feedback in a short amount of time. Thanks everyone.
No argument that a macro lens with telephoto such as the 105mm f2.8 maco would be ideal. That lens is $900 on amazon. I hardly even take my DSLR out for some serious shooting anymore. I have at least 1,000 photos I took during my honeymoon this year that I haven't had time to edit. So, with time invested to phono diminishing for me, I don't think I want to spend more than $300 on additional photo equipment to take pics of my critters with.
I browsed ebay looking for some vintage macro and telephoto lenses but think various options all have their limitations compared to their modern equivalents. For example, most telephotos won't let me get closer than 4' from the subject. And, good macro lenses under my budget seem to have low aperature. So, that's why I started looking at diopters. And, the reason I wasn't looking at extension tubes because it seems to me that they allow you to get closer to the subject, but I don't want to get closer to the subject. 1.25" with my current lens is close enough for me. Ideally, I want to put my camera on a tripod about 2' away from the tank. Well, I read the newer diopters were supposed to be very "clean" such as the Canon 500D and some say the Hoya I linked above is just as good as the Canon.
But, I will continue to learn more about the options presented by you guys. Thanks for your inputs!