I'm torn here. On the one hand, I know full well what it's like to be the one keeping a small child out of things, and would resent it if the person not doing it all the time dismissed my concerns. On the other hand, as the owner of a number of fish tanks, I know how passionate people can be about the hobby and resentful someone might be at feeling forced to give it up.
If you could be convinced that the tank was not a hazard, would you be willing to keep it in the front room? I know a lot of people who do keep tanks and toddlers without incident. It takes some planning, but it can be done.
First of all, let me reassure you that it's pretty hard to knock over a tank of that size, assuming it is on a stand made for that purpose. Aquarium stands are designed not just to handle the weight (water weighs about 8 pounds per gallon), but also withstand lateral stresses (like someone bumping into the tank). A wrought iron stand might not be a good choice, since not only might a kid try to climb it, but there's no way to keep wires and things out of a toddler's reach. A cabinet stand (like the one in the link someone provided earlier) can keep wires and other equipment out of sight and reach, and a child latch placed on the door. It could be bolted to the wall for greater stability and peace of mind.
Is there a lid on the tank? I know a lot of people keep open topped tanks, but the idea of an open topped tank in a house with kids or pets gives me the willies! Some cabinet stands come with matching wood "canopies" which keep things even more contained.
If you don't think the tank can be made safe, or don't want it in that room for other reasons, can it be placed anywhere else in the house that your boy does not have access to? Or, would your husband be willing to downsize, and find room for a smaller tank?
If the only option is to get rid of the tank for the time being, reassure your husband that it doesn't have to be forever. And perhaps have a plan for how long "the time being" is. Or, if this would sweeten the deal, perhaps you could get your husband one of the "nano" tanks that are becoming quite popular now. Some come complete with lighting and filtration, and would fit on a kitchen counter or desktop. Maybe he'd like to explore a new aspect of the hobby, by keeping a nano reef tank, or a planted nano tank.
I got out of the hobby for a number of years after my first daughter was born. We were under a lot of stress, and caring for my tanks was becoming more a chore than a source of relaxation. I finally got back into it (with a 29 gallon) when my youngest was 3 (and my nephew and niece were 3 and 1). Except for peanut butter fingerprints (and the occasional nose print) on the front glass, we've had no problems. I have since set up quite a few more tanks (yes, I'm a bit obsessed), and share the hobby with my two younger daughters, who are now 11 and 8.
Yikes! 12 new replies since I started typing! I think I'll post anyway and hope for the best!