You are ever so welcome. The good news is I don't think you're going to need a heater. 18-20C is right where you want to be with D. auratus. Once you have lighting setup, your temps will rise approximately to 23C during the day which is right where you want to be during the day. 18 C is perfect night time temps for them. Dart's natural habitat becomes cooler at night so the temp drop is appreciated. If you get your hands on a highland morph, they tolerate lower temps down to just above 16C.

I would however stay away from plexiglass. The heat will warp it within a few weeks. Trust me go with glass. You won't be sorry. I'm sure prices are comparable up north. A glass lid cut for a viv that size usually runs around 10 dollars American. Bring measurements to your local hardware store and they can do it for you in no time. The front of the viv is already vented so you don't have to worry about the glass fogging too much. Go ahead and cover the entire top. The humidity will maintain 80-90% with very little effort especially since I think I see the beginnings of a water feature there.

Some advice, make sure you silicone the heck out of the background by the water feature so the little guys don't climb behind the background or you'll have to tear the whole thing to pieces.