very cool! Is the sphere glass or plastic?
Did you use something marketed as a fish sphere, or get creative? I've been wanting to try a light globe, but haven't found one big enough.

The sphere is acrylic and designed for an outdoor lamp. I got it from 1000bulbs.com but I saved $12 in shipping by buying through Amazon (weird).

I managed a slightly higher base using two metal plant stands. The trick is to get something strong enough to support the weight. When filled with water it is over 100 lbs and most plant stands just collapse. We cable tied two together and it seems strong enough.

The fish are even exploring it now without the lure of food! And it's amazing at night with a light in it.

I would definitely say that 18" is a good size, depending on your pond and fish, of course. My largest goldfish is about 8 inches and managed to get in and out without any trouble. The opening is 8" but the cross pieces on the plant stand make getting in and out a little tricky. But, of course, fish are smart so they get the hang ot is pretty fast!

That is great!! I remember when a few of us did that a few years ago. What fun.