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Glass & Gravel
A planted nano aquarium on a desk, lit from above
Betta & nano tanks

Right-sized gear for betta and nano tanks

A rated 5-gallon holds about 4.3 gallons of water and needs 15.1 watts to hold 78°F in a normal room. Nothing is sold at 15.1 watts, so the answer is a 25. We publish that sum on every page, and you can check it.

Work out your wattageHow we pick
Where we stand

Units we claim to have tested: zero. Everyone in this category says they tested twenty tanks. We have not tested any, and we say so.

What we did instead: read the manuals, compiled the published specifications for 20 products, and did the arithmetic they imply - 20 of them carry at least one figure we could trace to a manufacturer page, and where a maker publishes nothing we print that absence rather than a number we invented. The heater wattage on this site comes out of Fluval’s own published ratings. The turnover figures come out of your tank’s real water volume, not its shelf label. Every sum is on the page you need it on.

The full method, and the four things we refuse to claim.

Start with the decision

Six things a nano tank makes you choose

Each of these is a destination rather than a menu: what to buy first, how the category divides, what decides the price, and the mistake buyers make.

What wins, and what it costs

The picks, with the price on the button

Where we hold a live price it is inside the buy button. Where we do not, the button says “check price” - never a stale number we cannot verify today.

The shortcut

What size heater does your tank need?

Take your rated volume, subtract 15% for substrate and the fill line, multiply by 0.35 watts per gallon per degree of lift, and round up to a size somebody actually sells. Every figure in that sentence is derived from published manufacturer ratings, and the derivation is on the page.

TankWaterRequiredBuy
3 US gal2.6 gal9.1W10W
5 US gal4.3 gal15.1W25W
10 US gal8.5 gal29.7W50W

Shown for a room that drops to 68°F overnight. Colder rooms need more.

See the full table and the derivation
Read these first

The four pages that carry the argument

A notebook and calculator on a desk, used for working out equipment sizing
Guide

What size heater your tank actually needs

The full derivation: where the watts-per-gallon-per-degree constant comes from, what we assume about substrate, and a table for every common tank size and room temperature.

Water being measured into a clear glass vessel
Guide

How many gallons does a betta need?

Five gallons is the floor and the reason is buffer, not swimming room. What each volume actually buys you, and the threshold where tank mates become possible.

Small tropical fish swimming among plants in an aquarium
Guide

Betta tank mates, threshold first

The volume threshold stated before the species list: invertebrates only at 5 gallons, fish from 10, and the honest fact that a betta needs no company at all.

Water test tubes used for aquarium water testing
Guide

How to cycle a betta tank

The fishless cycle step by step, with published bacteria dosing converted to nano volumes, the readings to expect each week, and how to cycle with a fish already in the tank.

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How this is funded

Glass & Gravel is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program. We earn from qualifying purchases made through links on this site, at no extra cost to you. It is the only way the site is funded: there is no sponsored placement, no paid review, and nothing here has been supplied free by a manufacturer.

A commission never moves a ranking. Several of the things we recommend most strongly - a plain ten-gallon glass tank, a sponge filter, feeding less - pay us little or nothing, and we recommend them anyway because they are the right answer.

The full affiliate disclosure, and our editorial policy. The badge on this page reads “Specs checked · 2026” because checking published specifications is what we actually do.