Glare and reflections off glass can be dealt with as described by Jacky.
Wear dark clothes when using a digital camera to photograph fish/ fish tanks because digital cameras pick up glare much more than film cameras.
You can use black card on the back of tanks if you want a black background. If you want to paint the back, you can paint wood, rocks, plants and other things on the glass and when it's dry, put a coat of black over that. Then you can have a scene on the back covered with a black background.
Don't use turps/ oil based paints if the tank is set up because the fumes can affect the fish.
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Discus, rummynose tetras and lemon tetras don't naturally occur in areas with rocks. If you want to keep the rocks in the tank, make sure you gravel clean in and around them to remove any gunk that is hiding between them.
These fish also dislike bright tanks and light coloured substrates. They naturally occur in blackwater (tea or coffee coloured water) and not in brightly lit areas. Some floating plants would reduce the glare off the back and bottom of the tank and make the fish feel safer and more secure. Changing the backing to a darker colour would also help, and the fish would show better colours. Lots of live plants would also help the fish feel more secure and show better colours.
I would remove most of the rocks, leave a couple of big ones if you like. Fill the tank with live plants and have some floating plants like Water Sprite (Ceratopteris thalictroides/ cornuta) or duckweed. Change the backing to something darker (blue, green or black).
Have the temperature around 26-27C and if the fish get sick, you can increase it to 28-30C. You can insulate the base, back and sides with 1-2 inch thick polystyrene foam sheets to reduce heat loss and power consumption. A 4, 5 or 6mm thick coverglass will also help reduce heat loss.
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I have some concerns about one of the red discus on the left side of the tank. Three of the fish seem to hang out together but the other fish seems to isolate itself. This could be caused by illness or bullying from the other 3 discus. The forth fish also looks a bit skinny in the back and this is usually diet related and bullying. However, it could also be from the angle in the picture.
When wild discus live in groups during the dry season, there are generally lots of fish living together. They have a pecking order and the smaller weaker fish sometimes get picked on by the other fish in the group. However, any aggression is spread out over all the fish in the group and the smaller weaker fish aren't badly affected by any bullying that might occur. If things get bad for the smaller fish they can swim to another hiding place. In an aquarium with only a small number of fish, the bigger more dominant fish can and do bully smaller weaker fish and these subordinate fish often waste away due to stress. In the confines of an aquarium the smaller weaker fish can't swim away and remain trapped with the bully until they are removed or die.
As a general rule, discus should either be kept in breeding pairs or in groups of 6 or more individuals that are similar size.
I don't think the tank is big enough for 4 adult discus. Discus can grow to 10-12 inches in diameter (although 6-8 inches is more common in captivity), and a mature pair need a tank that is 4 foot long x 2 foot wide x 2 foot high (or bigger). More fish need more room.
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You need live plants in the tank for the discus. Contrary to most literature out there, much of which has been provided by Jack Wattley, discus are primarily vegetarian. The reason people have trouble with discus developing intestinal problems is due to a high meat based diet and a long digestive tract. The meat spends too much time in the fish's intestine and they develop internal bacterial infections caused by the rotting meat.
The following link has a research paper that was done by scientists in 2008 and sheds some interesting light on the subject of the discus fish's diet. A brief summary of the paper says their diet in the wild consists of 55-85% plant matter. The rest of the diet includes small insects and crustaceans.
https://moam.info/the-discus-fish-symphysodon-scielo_59c411371723ddc7f4532afc.html