I have a number of small, rimless tanks, and I love the look. But I got very tired of the amount of water that evaporated out of them. It also limits the fish you can keep, because jumpers… jump. Bailin Shaw (Aquatic Gardeners Association and Chicago Aquatic Plant Society) shared this this simple, elegant solution with me. I’ve covered all my nanos this way since!
Have a piece of glass cut so that it fits RIGHT to the OUTSIDE edges of the tank. (measure carefully) Make it enough shorter than the tank measurements that your filter and cords will fit behind it.
Then use either sand paper or a sanding sponge (I found the later easier to use on glass edges) to file down the sharp edges of the glass. Take your time and do a thorough job, including the corners. If you don't, DON'T complain to me that you've cut yourself on it!
Then stick a silicone furniture bumper on each corner, in from the edge JUST far enough that the bumpers sit inside the tanks walls. Here's a link to the ones I ordered on Amazon, but I'm sure you can get them locally too.
I prefer the square ones because they really hold the cover securely, but you can use round ones if that's all you can find.
I find that the little space in the back is large enough to feed my fish without lifting the cover, but it isn't big enough to allow a lot of evaporation, and no one has found their way out.
With any glass cover on a planted tank, it is imperative to keep it really clean, or you will reduce the light getting to your plants. Use a damp paper towel to wipe dust off a couple of times a week, and when you do your weekly water change, rinse and wipe down the whole cover. If you start to get any lime build-up, wipe it down with white vinegar. If it's bad, they are cheap to make, so just replace it!
'Since I originally wrote this, I found nice little clear glass handles on Amazon. These give a nice, clean look to the tank while also making it easier to lift up for feeding and maintenance.