Tanks and kits, sized by what they actually hold

Almost every argument in this hobby about tank size is really an argument about two different numbers wearing the same label. A tank sold as five gallons is five gallons of glass. What it holds once you have put an inch of substrate in the bottom and stopped filling an inch below the rim is closer to 4.3 gallons. That is a fifteen per cent difference, and fifteen per cent is the difference between a heater that copes and one that does not, between a dose that is right and one that is a third too strong, and between a stocking plan that works and one that crashes in August.

So the first thing this hub does is refuse the shelf label. Every wattage, every turnover figure and every dose on Glass & Gravel is computed from actual water volume, using a stated assumption you can disagree with and redo: substrate, hardscape and the fill line take about 15% of rated volume.

What to buy, if you read nothing else

For one betta, buy a rated 5 gallons or more, and prefer length over height. A betta is a horizontal swimmer with a small territory and a labyrinth organ that sends it to the surface to breathe; a tall narrow column gives it a long climb and no room to patrol. The Fluval Spec V is here at the top of the hub for exactly one reason: at 19.5 inches long it is the longest footprint anyone sells at five gallons.

If you can fit ten gallons, fit ten gallons. It is not that a betta needs ten; it is that ten is where tank mates stop being a gamble and where temperature stops swinging every time somebody opens a window. The honest version of the advice is that a 5-gallon is enough for the fish and a 10-gallon is enough for the fish and everything you will want to add later.

How this category divides

There are really three kinds of small tank on sale, and they are not competing with each other.

Bare glass boxes. A tank, a lid, nothing else. Cheapest per gallon by a wide margin, and the right choice if you already know what filter and heater you want. You are buying glass and silicone, and glass and silicone have barely changed in thirty years.

All-in-one kits with a rear chamber. The Spec and Flex families, and their imitators. The pump and media live in a hidden compartment at the back, so the display is clean and there is nothing hanging off the rim. You pay a premium for the design and you lose some display volume to the chamber - the Spec V is 19.5 inches of glass but only rated five gallons for precisely that reason.

Bundle kits. A tank plus a clip-on filter plus a light plus sachets of food and conditioner. These look like the best value on the shelf and usually are not, because the filter is a cartridge design that you rebuy forever and the light is bright enough to grow algae and not much else. The exception is when the bundle is genuinely the cheapest route to a big enough tank - a ten-gallon bundle beats a five-gallon designer kit for a betta every single time.

What decides the price

Not the glass. Low-iron glass, rimless edges and mitred corners cost real money and genuinely look better, but the bulk of the price difference between a $40 tank and a $120 tank is the integration: a hidden filter chamber, a fitted lid, a bonded light. You are paying for the equipment to disappear.

The second driver is the light. A kit that publishes lumens and color temperature - the Spec V publishes 821 lm at 7000 K - is telling you it expects you to grow plants. A kit that publishes neither is telling you the light is decoration.

The mistake buyers make

Buying the kit and assuming it is complete. Not one of the nano kits in this hub ships with a heater, and a betta needs one everywhere in the continental United States. Budget for the tank plus a heater plus a thermometer, and if the kit’s pump is strong - the Spec V’s runs 55-80 GPH into 4.3 gallons of water, which is thirteen to nineteen turnovers an hour against the four we size for - budget for baffling the outflow too. None of that is expensive. All of it is invisible on the shelf.