How to size your filter, heater & lighting to your tank

Every spec below keys off your actual water volume, and that's almost always less than the number on the box. A tank loses gallons to substrate, rock, decor, and the inch or two of air you leave at the top. A "29-gallon" planted tank with a couple inches of gravel and driftwood is really holding closer to 24.

So before you shop, get the real figure. Length times width times height in inches, divided by 231, gives you gallons to the brim. Then knock 10 to 15 percent off for substrate and hardscape. If that's more arithmetic than you want to do, bring us your tank's three dimensions and we'll do it on the spot. Every part below gets sized off that one honest number.

Filters are rated in gallons per hour (GPH), which is how much water they push. The target most freshwater tanks want is four to six times the tank's true volume turned over every hour. A 40-gallon tank wants roughly 160 to 240 GPH. Go higher for messy, heavy-eating fish like goldfish or large cichlids, and ease off for a calm, lightly-stocked planted tank or shy fish that hate current.

One thing the box won't tell you: real-world flow is always lower than the printed number, because media clogs as it does its job and lift height (head) cuts a pump's output. So buy a filter rated above your minimum turnover rather than right at it. One that's a touch oversized and dialed back beats one that's maxed out and still falling behind. An oversized filter is rarely a problem; an undersized one is the one that lets the tank slide.

Turnover tells you how much flow; the tank tells you which kind of filter. For nano up through medium tanks (roughly 5 to 55 gallons), a hang-on-back (HOB) power filter is the easy answer. It clips on the rim, it's cheap, and it's simple to service. We stock HOBs from little 10-gallon units up to ones rated for 70-plus.

For medium-to-large tanks, heavily planted setups, or anyone who wants their gear out of sight, a canister filter is the upgrade. It sits in the cabinet, holds far more media than a HOB, and runs near-silent, which makes it the standard choice for a planted 55 or 75. For big tanks and any reef, you graduate to a sump: a second tank under the stand that adds water volume and room for a protein skimmer. We've written both of those builds up. See our freshwater sump guide for a no-drill 20-to-75-gallon version, and our saltwater sump guide for a full 125 reef build.

Heater wattage scales with volume. Figure roughly three to five watts per gallon to hold a normal 76 to 78°F. The reason it's a range and not one number is your room. A tank in a warm living room sits near the low end, around 3 watts per gallon; a tank in a cold Erie basement, a drafty back room, or near a winter window needs the high end, closer to 5, just to keep up. The bigger the gap between room temperature and tank temperature, the more wattage you need.

So a 20-gallon in a heated room is happy on a 75-watt heater, while that same 20 in a chilly basement wants 100. A 55 lands around 200 to 275 watts depending on the room. Always round up to the next size made. An oversized heater barely runs; an undersized one runs constantly trying to catch up and burns out early.

Once you're past about 50 gallons, don't run one giant heater. Run two smaller ones that add up to your target wattage, placed at opposite ends of the tank. A 75-gallon needs somewhere around 300 watts in a warm room and closer to 375 in a cold one, so two 150-to-200-watt heaters cover either case better than a single big unit.

There are two reasons for the split. First, a single stuck-on heater on a big tank can cook everything before you notice. Split the wattage across two, and neither one alone has the power to do that, while a stuck-off failure only drops you halfway instead of all the way. Second, two heaters at opposite ends spread the warmth evenly instead of leaving a cold corner. On any tank you care about, we'd also put the heaters on a separate temperature controller so a failed thermostat can't run away with the tank.

Light is the one spec where gallons matter least and your goal matters most. There are three honest tiers, and buying above your tier just grows algae.

If you keep fish and maybe a few hardy low-light plants like anubias and java fern, a basic full-spectrum LED is all you need: enough to see the fish and show their color, nothing more. If you want a real planted tank with stems and a carpet, you need a plant light with usable PAR (the light intensity plants actually photosynthesize with), sized to your tank's length and run on a timer. That's the difference between thriving stems and a melting mess, and our low-tech planted guide covers the rest. Growing coral is a different world again. Corals demand high-PAR reef lighting heavy on the blue and violet wavelengths their symbiotic algae use to photosynthesize, and the light is one of the most important and most expensive pieces in the build. We carry all three tiers; tell us your goal and we'll point you to the right one.

Here's the whole thing at a glance, by bracket. NANO (5-10 gal): a small HOB rated for the size or a sponge filter, a 25-to-50-watt heater, and a basic LED. Keep it simple; nano tanks swing fast, so don't over-gear them. SMALL (20-29 gal): an HOB around 150-200 GPH, a 75-to-150-watt heater (basement tanks take the higher number), and a light matched to your goal.

MEDIUM (40-55 gal): step up to a larger HOB or your first canister, a 150-to-275-watt heater, and a proper planted light if you're going planted. LARGE (75-125+ gal): a canister or a sump, and now you split heaters into two units totaling 300-to-450-plus watts, plus a light sized to your full tank length, and reef lighting if there's coral in the plan. Treat these as starting points and adjust from there. Stocking, room temperature, and your goals all nudge them, which is exactly why we'd rather size it with you than have you guess off a chart.

Sizing a filter, heater, and light isn't hard, but it's easy to get a little wrong in a way that costs you months of algae or a chilly tank. The fastest fix is to skip the guessing.

Bring your tank's length, width, and height in to the shop at 660 East 14th Street, or call us at (814) 456-9445, and we'll work out your true gallons and match a filter, heater, and light to your stock and your room, right down to whether your spot in the house needs that extra wattage. Once the tank's running, bring a water sample by anytime and we'll test it for you. We'd rather spend ten minutes getting it right than sell you a box that says the right number on the front.