Unfortunately, electric heaters are already at 100% efficiency, there's no way to improve performance and save money on the heat-generating side of keeping the fish tank warm.
You might be able to reduce heat demand by insulating the bottom, back, and sides of the tank, so there is less exposed surface area to let heat escape to the room. I'd be hesitant about covering up the top, for fear of reducing oxygen flow, though.
If you hold your hand near the older windows when the wind is blowing and feel airflow, you could get a caulking gun and some tubes of caulking at a hardware store, and fill the gaps around the glass. The caulking might not flow smoothly now that it's cold, though.
Do you have any drafts under your doors? If you're feeling crafty, a long tube of cloth filled with sand or kitty litter is a cheap way to reduce airflow.
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You might be able to reduce heat demand by insulating the bottom, back, and sides of the tank, so there is less exposed surface area to let heat escape to the room. I'd be hesitant about covering up the top, for fear of reducing oxygen flow, though.
If you hold your hand near the older windows when the wind is blowing and feel airflow, you could get a caulking gun and some tubes of caulking at a hardware store, and fill the gaps around the glass. The caulking might not flow smoothly now that it's cold, though.
Do you have any drafts under your doors? If you're feeling crafty, a long tube of cloth filled with sand or kitty litter is a cheap way to reduce airflow.
https://www.etsy.com/ca/listing/1187591464/draft-stopper-door-window-draft-stopper