Figure-eight pufferfish brackish tank setup

Figure-eight puffers are a fantastic choice — but they're one of those species where getting the water chemistry right before you add any fish is absolutely critical. The most important concept here is the nitrogen cycle: your tank needs to establish a colony of beneficial bacteria that convert toxic ammonia into nitrite and then into relatively harmless nitrate. Skipping this step kills more new fish than anything else. With figure-eights specifically, you've also got the added layer of brackish water — they need a low specific gravity, typically around 1.005–1.008 SG, which means your cycle needs to happen in already-salted water, not fresh water you adjust later. Are you starting from scratch, and what size tank are you thinking?

Good instinct on the size — 60 litres is a reasonable minimum for one figure-eight, and honestly two would be pushing it unless you're very on top of water changes, as they can be territorial. One thing that catches people out is cycling in brackish water: the beneficial bacteria still colonise fine at low salinity, but you need to be mixing your salt into the tank from the very start of the fishless cycle, not after. What are you planning to use to cycle — a ammonia source, or were you thinking of adding fish straight away?

Fishless cycling means you dose the tank with pure ammonia to feed the bacteria before any fish go in — it's much kinder and actually faster than fish-in cycling. You'll need a reliable liquid test kit to track ammonia, nitrite, and nitrate levels throughout the cycle, and this is genuinely the one thing I'd say don't skip or cheap out on. The API Freshwater Master Test Kit is the go-to recommendation here — it works perfectly for brackish tanks at figure-eight SG levels, it's far more accurate than strip tests, and it gives you ammonia, nitrite, nitrate, and pH all in one box. You'll use it constantly through the cycle and then for regular maintenance testing afterwards. For the salt, you'll want a marine salt like Instant Ocean rather than aquarium tonic salt — it buffers pH and adds the right minerals for brackish.

Typically four to six weeks, though it can be faster if you add a seeding source like a piece of filter media from an established tank. You're looking for a very specific endpoint: you dose ammonia to 2–4ppm, and within 24 hours both ammonia and nitrite read zero on your test kit, while nitrate has risen — that tells you both bacterial colonies are established and working. With the API kit you'll be testing every couple of days and watching the nitrite spike come and then fall, which is the satisfying part. Once you hit that clean 24-hour result, do a large water change, get your salinity dialled in with a hydrometer or refractometer, and you're ready for your puffer.

A few worth knowing: their teeth grow continuously, so they need hard-shelled food like snails, clams, or mussels regularly — not just pellets — or their beak overgrows and causes serious problems. They're also smart and get bored, so tank enrichment and varied feeding matters more than with most fish. And do get a refractometer rather than a basic swing-arm hydrometer for checking salinity — they're far more accurate and not expensive. But the API Master Test Kit is genuinely your most important first purchase. Get that, start your brackish fishless cycle, and you'll be in a really strong position by the time your figure-eight goes in.