What goes in a 10 gallon without turning it into a washing machine? Aquarium

One thing to plan for: the air pump will be the noisy part and everyone forgets that when they recommend sponges. Sit it on foam, keep it below the water line only if you have a check valve fitted, and buy a pump rated for more than one tank so it can run quietly at low output instead of screaming at full.

Sponge filter on a small air pump, and it is not a compromise — for a nano tank it is the recommended answer, not the budget one. Flow is gentle by nature, shrimp cannot get pulled in, and you clean it by squeezing the sponge in a bucket of tank water, which leaves the bacterial colony essentially intact. Total spend should be well under your budget and the only ongoing cost is the odd length of airline.

If you want to keep a hang-on-back for the surface agitation, the fix for flow is a baffle rather than a different filter. Cut a strip from a plastic bottle, clip it over the outflow so the water spills sideways down the glass instead of falling, and the current at the bottom of the tank drops off a cliff. Cost is zero and it took me five minutes.

On the ratings: filters are rated optimistically and actual flow through media at real head height is usually well under the number printed on the box, so a unit rated for 20 gallons on a 10 is less absurd than it sounds. That said, a betta does not need turnover, he needs oxygenation and biological media, and a sponge does both without the noise.

Ten gallons with a betta and shrimp is a light bioload with plants doing half the work. I ran mine on a sponge for four years and the only maintenance was a squeeze once a month and replacing the airline when it went stiff. Do not overthink this one.