The conventional wisdom for first-time fishkeepers goes something like this: "start small to keep costs down, learn the basics, and upgrade if you stick with the hobby." It sounds sensible. It's also wrong on the most important axis — small tanks are far harder to keep stable than large ones, and the chemistry of a 30-litre starter cube is brutal on a fishkeeper still learning what an ammonia test even means.
The real recommendation, from someone who has set up tanks ranging from 15 litres to 240 litres: buy the largest tank you can comfortably fit, afford, and lift while empty. Then check the seven specifications below before you hand over any money.
Why bigger is more forgiving
Three reasons, all rooted in chemistry rather than aesthetics:
Dilution. A pollutant — ammonia from a missed water change, leftover food that decays into a small spike — is diluted by the volume of water in the tank. A 30-litre tank with 10 ml of accidental fish food turning into ammonia is a far more concentrated problem than the same 10 ml in a 200-litre tank.
Thermal stability. Larger volumes of water change temperature more slowly. A heater failure in a small tank can cause a temperature crash within hours; the same failure in a 200-litre tank gives you a day or more to notice and intervene.
Surface area for gas exchange. Larger tanks, particularly long-and-shallow designs, offer more surface for oxygen to enter the water. This buffers against filter failures and overfeeding incidents that would suffocate fish in a smaller container.
The threshold I'd put for a true beginner: 75 litres minimum, 120 litres or more strongly preferred. Below 75 litres, the chemistry is too tight; above 120, you're in a comfortable margin that forgives most beginner mistakes.
Why footprint matters more than capacity
Two tanks of identical 100-litre capacity. One is 100 × 40 × 25 cm (long and shallow). One is 40 × 40 × 62 cm (tall and narrow). The first is a far better aquarium.
- Surface area. 4,000 cm² versus 1,600 cm² — the long tank can support 2.5× the gas exchange.
- Swimming length. 100 cm versus 40 cm — most active species need length, not depth.
- Hardscape and aquascaping room. A wider footprint gives you space to lay out plants and decor in a way that creates territories and sightlines. A column tank tends to get a single feature rock and a lot of empty water.
- Lighting and maintenance. A shallow tank means light reaches the substrate (good for plants) and your arm reaches the bottom (good for cleaning).
Tall, narrow "designer" tanks photograph beautifully and look striking on a stand. They are also significantly worse aquariums for the fish inside them. If aesthetics are driving the choice, look at low-iron rimless tanks with footprints of at least 60 × 30 cm.
The seven-point checklist
What I actually check before buying any aquarium, from a chain-store starter kit to a custom-built tank:
1. Internal vs external dimensions
Manufacturer specs almost always give external dimensions. Subtract roughly the glass thickness × 2 from each axis to get the internal volume — and use our tank volume calculator with the glass-thickness option to do the maths properly. Real water volume on a "100-litre" tank with 8 mm glass is closer to 92 litres before substrate.
2. Glass thickness vs water height
Tank glass should be sized to the water depth: roughly 4–5 mm for tanks under 30 cm tall, 6 mm up to 45 cm, 8 mm up to 55 cm, 10 mm up to 65 cm, and 12 mm beyond. Anything thinner than these for a given height is a leaking-tank-in-three-years risk. Ask the seller, or measure with calipers if you can.
3. Bracing and rim quality
On rimmed tanks, check that the rim sits flat with no gaps. Inspect the corner silicone — it should be smooth, gap-free, and roughly the same width along all seams. On rimless tanks, check that the top edges are polished or chamfered (sharp glass edges chip and crack).
4. Stand or cabinet rating
A litre of water weighs about 1 kg. A 200-litre tank with substrate, decor, and equipment weighs comfortably over 250 kg. Most flat-pack furniture is not rated for that. Either buy a purpose-built aquarium stand, an MDF/melamine cabinet specifically marketed for aquariums, or build/commission one. Marble, granite, or thick hardwood furniture that is rated for the load also works. Cheap pine bookshelves do not.
5. Filter throughput
If the tank comes with a filter (most starter kits do), check its rated turnover. You want 5–8× tank volume per hour. A "100 L tank with 200 L/h filter" kit is undersized for any real stocking. Plan to upgrade the filter immediately, or buy the tank without the filter and source one separately.
6. Lighting
Stock lighting on starter kits ranges from "barely adequate" to "actively bad." For a fish-only tank with hardy or no plants, the bundled LED is usually fine. For any plants beyond the toughest (Anubias, Java fern, mosses), you'll want roughly 0.5 watt per litre of LED, or one of the modern aquarium-specific LED units sold separately. Don't underestimate this — bad lighting kills plants faster than bad water.
7. The location it's going to live in
Often forgotten until after the tank is full of water. Check:
- Floor loading. Upstairs floors in older houses can struggle with anything above 100 litres along a single joist. Place tanks parallel to floor joists, not perpendicular, and consult a structural engineer for tanks above 200 litres on suspended floors.
- Direct sunlight. Avoid. Algae thrives. Temperature spikes ten degrees on a sunny afternoon.
- Power sockets. A tank typically needs three to four outlets — filter, heater, light, possibly air pump. Add a dedicated extension if the location has fewer.
- Spillage tolerance. Tanks leak occasionally. Floors that don't tolerate water (untreated solid wood, electronic equipment below) are bad locations. A tile, vinyl, or sealed-stone floor is ideal.
What's worth spending on, what isn't
If you have £300 / $400 to set up a first tank, here is roughly how I would allocate it:
| Item | Budget allocation | Notes |
|---|---|---|
| Tank (100–120 L rectangular) | ~30% | Quality glass, good silicone, rated for the use. |
| Stand or cabinet | ~15% | Don't skimp. A failure here is catastrophic. |
| External or internal filter (rated 600–800 L/h) | ~20% | Most-upgraded item in starter kits — buy good once. |
| Heater (with controller) | ~8% | Get one with an external thermostat or controller — heater failures are common. |
| Lighting | ~7% | Sufficient for fish; upgrade later if going planted. |
| Substrate, decor, plants | ~12% | Buy plants over plastic. They're cheaper long-term and they help the chemistry. |
| Water test kit | ~5% | Non-negotiable. API Master Test Kit or equivalent. |
| Dechlorinator, fish food, net | ~3% | Boring but essential. |
What I wouldn't spend on, at least initially: ornate tank decor, branded accessories, second filter "for redundancy" (one good filter beats two cheap ones), automated dosing systems, anything sold in a "starter pack" that bundles in items you don't need.
Tanks I'd recommend looking at
I won't name specific brands or models — they change too fast and I don't take review samples. Categories worth shortlisting:
- Rectangular rimmed tanks 90–120 cm long. The classic 75–150 litre community tank size. Cheap, structurally sound, and the geometry suits a wide range of stocking plans. Most major brands make a version.
- Rimless low-iron tanks in similar dimensions. More expensive, more attractive, slightly more fragile. Worth it if budget allows.
- Aquascaping-focused tanks (the "shrimp tank" / "iwagumi" form factor). Often 60 cm × 30 cm × 36 cm or similar. Smaller than I'd suggest for a true beginner, but a good second-tank purchase for someone who wants a planted display.
Avoid: bowls, columns, tall hexagons, any tank under 40 litres for a first purchase, and starter kits that bundle in multiple goldfish (the goldfish need at least 75 litres each).
One last note on used tanks
A used aquarium from a Facebook group or eBay can be excellent value — often 40–60% of the new price for a tank that's structurally identical. Things to check before buying:
- Fill it with water (in the seller's house, ideally) and watch for leaks. Particularly along the bottom seam.
- Inspect the silicone — it should be flexible and well-bonded, not flaking or yellowed.
- Look for chips on the rim and corners. Small chips are cosmetic; cracks running along an edge are not.
- Ask why it's being sold. "Upgrading to bigger" is a reassuring answer; "leaks occasionally" is not, no matter how cheap.
Used filters and heaters are a tougher call — both have wear parts, and the savings often aren't worth the failure risk on a brand new setup. Used substrate I'd never buy unless the source is impeccable.
Closing
The right first aquarium is large enough to be forgiving, shaped for the fish that will live in it, and located somewhere it can stay for years. Get those three right and most of the early-fishkeeping headaches simply don't happen. Get them wrong and you'll spend the next twelve months wondering why the hobby is so much harder than the magazine articles suggested.
Once you've chosen the tank, run your real water volume through our calculator, plan your stocking with the stocking math article, and cycle the tank using the fishless cycling protocol before any fish go in. That sequence — buy carefully, calculate honestly, cycle properly — is the difference between a hobby that lasts and a starter kit that ends up in the loft.