A domestic UK socket can supply 13 Amps (Just over 3kW) so the fire is the only thing that should be connected if it is being used at full power. In truth the other items you mention add negligible additional load so you will probably get away with it.
You say you have reset it using a switch so you have an RCD (Residual Current Device) which is quite sensitive.
click here would disconnect all the appliances from the sockets, except for the fish tank heater of course, and see if the problem repeats. If it does, then the problem is either internal house wiring or the fish tank heater so disconnect the heater or replace it and see what happens.
If it does not repeat then reintroduce each appliance one at a time and monitor the situation.
In all honesty you will probably find that it is fine but worth keeping an eye on.