What makes a "Freshwater" aquarium? Freshwater aquariums are manufactured to accept several types of freshwater filtration. The top configuration is set up to accept hang on the back filters, canister filters and sponge filters. The body of the aquarium and materials used are the same as all the rest of our acrylic aquariums and filters. If you have any special needs for a custom top configuration feel free to contact us and we'll see if we can accommodate your request.