Aquarium In-Line & Filter Heaters
Aquarium filter heaters are a relatively new player on the market, so there are fewer choices and they are somewhat pricey. But they offer distinct benefits. Aquarium filter heaters install into your filter or sump, so all the water in your tank gets circulated through, providing even heating not always possible with other heaters. They are the most aesthetic choice since they're completely external. And your filter protects against any damage that might occur in the open aquarium.
The ETH models fit canister filters or sumps. If you have the budget for it, aquarium filter heaters are a great heating choice with many long-term benefits.