Do Medaka Need a Filter?

Written by Chris, the breeder at Firefly Medaka

Often, no. A lightly stocked, well planted medaka container runs perfectly well with no filter at all, which is one of the reasons these fish suit a patio tub so well. But there are situations where a filter genuinely earns its place, and one kind of filter is clearly the right choice when it does.

Why medaka can go without

Three things make a filterless medaka container work. They are small fish with a low bioload, so there is not much waste to process in the first place. Floating plants consume nitrogen directly from the water, doing much of what a filter's biology would do. And a wide container has enough surface area for gas exchange without help.

Put those together and a lightly stocked, planted tub is a stable little system. That is not a compromise or a shortcut, it is how medaka have been kept in Japan for centuries, in shallow bowls with plants and nothing else.

From my setups: I run gentle sponge filters in my pots, and the fish genuinely seem to enjoy playing in the bubble stream. My containers are also stocked far heavier than a home tub, which is exactly the situation where a filter starts to matter.

When a filter is worth it

Add a filter when any of these are true. You are stocked densely, beyond roughly one fish per two gallons. Your container is small or narrow, so surface area is limited. You have few or no live plants. Or the tank is indoors, where there is no rain, no natural top-up, and less light for plants to work with.

Indoor tanks are the common case. An indoor ten gallon with eight medaka and a couple of plants is a tighter system than the same eight fish in an outdoor tub, and a small filter takes the pressure off.

If you use one, use a sponge filter

The type matters more than the brand. A sponge filter run off an air pump is the right tool for medaka for three reasons: the flow is gentle, the sponge grows exactly the bacteria that process waste, and crucially fry cannot be sucked into it. A hang-on-back or canister filter with a strong intake will eat baby medaka, and it pushes more current than these fish want.

Medaka come from still and slow water. They are not strong swimmers against current, and a fish being pushed around a tank all day is a stressed fish. If you use a filter with real flow, aim the output at the glass or a wall to break it up.

What a filter does not replace

A filter processes waste, it does not remove it. Nitrate still accumulates, which is what water changes and live plants are for. Filters also do not fix overfeeding or overstocking, they just delay the consequences.

Plenty of keepers reach for a filter when the actual answer is fewer fish, less food, or more plants. If you are choosing between spending on a filter and spending on floating plants for a tub, the plants will usually do more.

The short version

Outdoors, in a wide tub with floating plants and sensible stocking, skip it. Indoors, or stocked heavily, or with few plants, add a gentle sponge filter and you have covered the gap. Either way you do not need a heater, which my heater guide explains, and that combination is why medaka are one of the least demanding fish you can keep.

Simple setups welcome

Fish that thrive without equipment

Medaka need no heater and often no filter. Every live order ships with free floating plants, which is the piece that does most of the work in a filterless tub.