buy a group of Pseudomugil signifer rainbowfish and keep them in the 65litre. They grow to about an inch long and breed readily. They don't even normally bother their eggs or young. You can have plants in there and they would be happy.
Alternatively Melanotaenia praecox is another rainbowfish. They grow a bit bigger and do eat young but and quite nicely coloured.
The above are both tropical fishes so you would need to keep the water warm. I am not sure why you are affraid to use the heater in the tank. They are usually quite safe unless the glass is cracked or there is moisture inside the heater. Having a circuit breaker (trip switch) on the power point for the fish tank will prevent you from getting electrocuted should something go wrong. They can also be fitted into the main fuse box in the house which is a better place to have them.
Gold Medakas are a nice coldwater fish and so are bitterlings. Bitterlings lay their eggs inside mussels and the babies swim out a month or so later.
Go to the petshop and make a list of the fish you like. Then you have somewhere to start.