I can understand marine tanks using a lot of power, but freshwater, seriously? They use hardly any electricity, so why would your parents be concerned about more tanks using too much power? If it was a fish room then that would make sense, but a few tanks is nothing for them to be worried about
My parents aren't worried about the cost of electricity, but the fact that the tanks are in my room. If we had a bigger house I'd be allowed a fish room, and mum would really love me to get a reef tank too because she loves the invertebrates and corals and the colours of saltwater fishes