I have an in-wall 75 gal. tank. For many years it was a planted display tank. it housed my clown loaches before they got big. And then one day it developed a leak in the bottom from right corner area. Fortunately it was a slow leak and I was able to move all of the contents into the 150 gal. there all now reside.
I do not repair leaking tanks, I replace them. I did this with the 75. However, I never got it set up as a display. I put it into use as a "junk tank." I put in a thin layer of sand but any and all plants that went in were in pots or on wood or rocks. One of the first uses for the new 75 was to Q 40 red line barbs at 2 inches. These were to be sold at the NEC about 6 months later.
Since then the plant load has doubled or maybe even more. The barbs are no longer in the tank, In its current incarnation it mostly holds a mix of fish I bought to resell at the NEC, and those that did not sell I kept for myself. The tank now holds paleatus and albino corys, rummy nose tetras, orange fin danios, and purple emperor tetras plus way too many amano shrimp and assassin snails.
The tank lights are on a timer and come on at about 10 a.m. However, when I get up and open the blinds and the sun comes up, it will shine into the front right corner of the tank. As the sun climbs, the portion of the tank that is fully illuminated by sunlight moves from right to left. Despite staring at that tank whenever I am at my desk working or on my PC, the tank is about 9 ft. from me.
I see this tank all day long but c couple of days ago I notices something which I confirmed today. When that small area on the lower front right of the tank gets the sun, the rummy nose tetras are all in that area. The rest of the tank is still pretty dark. What I found rally interesting was how those tetras behaved. The would swim to the place where the line between the light and dark was and stop. They would stare into the dark and then turn around. They seemed to be scared of the dark- almost like it was a deadly radiation zone like we see in some futuristic movies.
Moreover, as the sun rose further it expanded the light area and the light/dark boundary moves left towards the center of the tank. As it does, the tetras expand the area in which they swim. They still stop where the light/dark boundary is. It did not take me long to figure out what this was all about. In the wild most fish like tetras sleep at night. During this time they are at risk from nocturnal predators. As the sun rises and begins to make it possible for the fish to see, they will venture from where they shelter at night into the lighted places. But they will not venture into the dark as they cannot see any potential predators there.
For me this was another learning first in a long lone of first over the years. It is this sort of thing which makes this hobby so much fun. No matter how long one has been at it, there is always more to see and to learn.