Actually, you even though you have 4 adult canas in there, you might possibly be able to house a small goldfish in tere for a few months or more. At least until it got about 5-6 inches max. Then, of course, it would need a larger tank anyway.
I am basing my opinion on the fact that you have double the filtration that is recommended for goldfish. The aqua-clear 30 pumps 150 gph and the fluval 1 pumps at 52 gph. That brings your total filtration to 20X'x an hour. With heavy filtration like that, I would imagine that stocking levels could easily be doubled.
But,
and I stress this, MANY large, regular water changes will be needed to keep the KH of the water at an optimal level and the nitrAtes low. With that much bio-filtration going on, you can bet that a lot of the KH gets used up and the nitrAtes will rise very quickly. Not to mention that you are talking about large snails. They too are known for using up all available kH in the water to assimilate their shells.
Hmmm, I wonder if that little guy is still there............
Good luck!