Hi and welcome to the forum
What are the tank dimensions (length x width x height)?
What is the GH (general hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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Synodontis catfish like to be in groups so if there is room, then get 4 or 5 of them.
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Oscars don't normally hide but the catfish do. Oscars hang out in the open and only use rocks and wood for breeding (to lay eggs on) and to help define territories.
If you have ornaments in the tank, make sure they are smooth and don't have any sharp edges.
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Oscars dig in the substrate and regularly uproot plants. You can grow floating plants like Water Sprite (Ceratopteris thalictroides/ cornuta). This plant grows rapidly on the surface and can get huge. I had one plant that filled a 4ft x 2ft x 2ft aquarium. You can also grow the plants in the substrate and if they get uprooted by the fish it won't affect the plant.
The other option is to grow Java Fern or Bolbitis on driftwood and rocks. These plants can be tied to wood or rocks with a bit of string, fishing line or even rubber bands. They eventually adhere to the rocks and wood and will stay there if the string breaks.
Amazon sword plants, Hygrophila species and narrow Vallis are pretty tough too and can be grown in pots. You get a 1-2 litre plastic icecream container (or something similar) and put an inch of gravel in the bottom. You put a thin layer of granulated garden fertiliser on the gravel. You cover the fertiliser with a 6mm (1/4 inch) layer of red or orange clay. Dry the clay out and crush it into a powder before using it. Fill the rest of the container with gravel and plant the plant in it.
You can then cut a piece of shade cloth or mosquito netting (available from places that sell material for clothing) and put it over the container and use a rubber band to hold it on the sides. Cut a X in the middle of the shade cloth or mosquito net so the plant can fit through. The netting will stop the fish digging the substrate out of the pot but still allow water through.
The clay stops the fertiliser leaching into the water and provides iron and other trace elements. When the plant roots hit the clay and fertiliser they grow like mad.
The pots can be surrounded by rocks or left to grow algae on them so they are less conspicuous. You can also smear a thin layer of silicon on the outside of the pot and stick gravel or sand to the outside so it blends in with the substrate in the tank.
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Oscars and some other big fish sulk when there are major disturbances in their home. If you do big regular water changes every week, the fish get use to them and don't care if you change 20% or 90% of the water.
Due to the Oscars diet (meat based with high protein), you should do a 75% water change and gravel clean the substrate every week. You should also monitor the nitrate level in the water and if it is going above 20ppm during the week, then do two 75% water changes per week.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
The easiest way to clean a big tank is to get a 2 litre plastic drink bottle and cut the bottom off it. Throw the bottom and the lid away. Attach a length of garden hose or clear plastic hose to the top of the bottle and run the hose outside onto the lawn. Use the bottle as the gravel cleaner and drain the tank onto the lawn.
To refill the tank, have a couple of big plastic rubbish bins or storage containers and fill them with tap water. Add dechlorinator and aerate the mixture while you are draining and gravel cleaning the tank. Once the tank has been drained, use a water pump and length of hose to pump the new dechlorinated tap water into the tank. You can make a U out of pvc pipe and fit it to the hose and the U can hang over the edge of the tank and stop the water splashing about when you fill it up.
Use a metal hose clamp to hold the hose on the U shaped pvc pipe.
Filters should be cleaned at least once a month and every 2 weeks is better.