Hi and welcome to the forum
Sometimes aquarium heaters that have been sitting around for a while start to malfunction and these are best replaced asap. A 60 litre tank should have a 100-150watt heater. I prefer to err on the bigger side (150watt) because it won't have to work as hard when warming the water and can be used on a bigger tank later on if you ever upgrade. The price between them will be negligible.
Make sure you get an adjustable heater and get a floating glass thermometer to go in the tank if you don't already have one. If you buy a new thermometer, take a number of them off the shelf and line them up next to each other. Look at the temperature on each thermometer and buy one that has the same temp as the others. Sometimes they are faulty and will have a different reading to most of the other thermometers so try to avoid the one that reads several degrees higher or lower than the majority of the others.
Rena heaters are good but there are plenty of other brands. Make sure any heater has at least a 12 month warranty, and is adjustable and completely submersible.
Have the heater as close to the bottom of the tank as possible. Put it on an angle so the cord is above the heating coil, which is at the other end to the cord. Have an airstone or filter outlet near the heater to circulate the warm water more effectively.
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The maximum water temperature for most fish is 32C, however some fish will tolerate higher temperatures if it is increased slowly over time and the fish have been bred in warm conditions over a period of years. I have had fish in 45C water and they were fine but they were an exception. Most aquarium fish die when the temperature goes above 32C.
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Keep an eye on the tiger barbs because they can nip the fins on other fish when stressed. Stress can occur when they are kept in small groups (less than 10), or if there is not enough room to move about (small tanks), or lack of hiding places.
The angelfish will probably need a bigger home in the future when it has grown.
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I am unsure if you emptied the tank and moved it to your house or if you still live with your dad and the tank has not been drained, moved and set back up. If the tank has been moved to a different property, keep the feeding down to once a day and monitor the ammonia and nitrite levels to make sure the beneficial filter bacteria did not die from the move.
If the tank is still in the same house then the filter bacteria won't be an issue.