Freshwater vs saltwater aquariums

A 20-gallon freshwater community and a 20-gallon reef share a shape. That is the end of the overlap.

Freshwater

• Tap water, dechlorinator, a heater, a filter.
• Cycle with ammonia. Keep nitrate down with water changes.
• Livestock is cheap enough that mistakes still hurt the fish, but they do not cost a car payment.
• Plants want light, and maybe fertilizer and CO₂. They do not want salinity.

This is the hobby this site is built for: beginner setup, fish profiles, compatibility, tools.

Saltwater (fish-only or reef)

• Mix salt to a stable specific gravity. You need a refractometer, not a vibe.
• Live rock, more flow, usually a protein skimmer on anything that is not a nano experiment.
• Lighting for corals is a different product category than a $25 clip-on LED.
• Livestock dies expensive. Compatibility is about aggression and whether the angel will eat your coral.

We do not keep a fake saltwater fish list. If you need gallons or a heater for a marine box, the volume and heater tools still do math. They will not tell you whether a yellow tang will fit.

Which one should a beginner start with?

Freshwater. A 20 long, a hang-on-back, a heater, liquid tests, and a school of tetras. If you still want salt after a year of that staying alive, you will know what “stable” feels like.

Brackish is a third thing. Fiddler crabs and some puffers need it. They are not drop-in community fish. See the crab profiles if that is why you are here.

Cost, honestly

A decent first freshwater tank is a few hundred dollars if you skip the “complete kit” and buy a real filter. A reef that does not crash is a different budget. If the shop is upselling live rock for your first aquarium, you are in the wrong conversation.