Looking ready for habitation. Very cool!
I recommend using black or whatever color acrylic paint and painting all the sides you want covered. What is nice about acrylic paint is that it is water-based, non-toxic, and can be removed easily in the future. You can use the black cardboard cover for the front at night.Something has been vexing me and I think I figured it out. This tank is in the living room. And sometimes people in the house stay up late. So I've been trying to think of a way to cover the tank at night to there's a period of darkness for the fish to rest. Throwing a towel or blanket over the whole thing doesn't seem like a good idea. I was actually pondering something like cutting out part of a refridgerator box to place over the tank and stand at night. Or making a similarly sized foldable hood for that purpose.
Then a much simpler solution hit me. I put black posterboard on as a black background. I could do the same for the sides. And then I could attach some mini clothespin like clips to the top of the tank and use them to clip a piece of blackout curtain over most of the top and front.
Thanks. I still have a few more things to add. I want to add a piece of pvc pipe fitting as a cave. I'll attach Anubias nana petite to the outside of it to make it a little more natural looking. And I want to add some Anubias bartieri coffeefolia to the other rocks in there to give the betta more to explore. Plus some nice broad leaves for him to rest on. I don't like the empty space behind the sponge filter so I'll see if I can get some more moneywort behind that. And maybe a floating betta log for another place for the fish to hide.Looking ready for habitation. Very cool!