Advanced Fish Compatibility Guide

Cycling a new tank

A new tank needs a colony of beneficial bacteria before it's safe for fish - this is the nitrogen cycle. Fish waste produces ammonia, which is highly toxic; bacteria convert it to nitrite (also toxic), then a second group of bacteria convert that to nitrate (much less harmful in moderate amounts).

This process typically takes 4-6 weeks. Test your water regularly during this period, and add fish gradually rather than all at once - a tank stocked to capacity on day one has no bacteria to handle the waste yet.

The most reliable way to cycle is "fishless cycling": dose the tank with a source of ammonia (a pure ammonia solution, or something like a piece of raw prawn left to decay) instead of using fish to produce it. Aim for an ammonia reading of 2-4ppm, then test daily. You'll typically see ammonia climb and hold for a week or two, then nitrite appear and climb as the first bacteria establish, then both fall back toward zero as the second group of bacteria catches up. The tank is fully cycled once it can process a 2-4ppm ammonia dose down to 0 ammonia and 0 nitrite within 24 hours, with nitrate present.

You can speed the process up by seeding your new filter with mature media, gravel, or filter floss from an already-established tank, or with a bottled bacterial starter product. If fish are already in the tank while it cycles, keep ammonia and nitrite as close to zero as possible with frequent small water changes, and hold off adding any more fish until the cycle is complete.

Understanding water parameters

Temperature - most tropical fish need a stable range, typically 22-28°C (72-82°F). Sudden swings stress fish more than being slightly outside the ideal range.

pH - a measure of acidity/alkalinity, from 0-14, with 7 being neutral. Most community fish tolerate 6.5-7.5, though some species need water outside that range.

Hardness (GH) - the concentration of dissolved minerals. Soft water suits fish from blackwater rivers (many tetras); hard water suits fish from mineral-rich lakes (many livebearers, African cichlids).

GH vs KH - General Hardness (GH) measures dissolved calcium and magnesium, the minerals fish and plants actually use; Carbonate Hardness (KH) measures buffering capacity, i.e. how resistant the water is to pH swings. Low-KH water can see its pH swing unpredictably even with good maintenance, which is why some experienced keepers test KH alongside pH rather than pH alone.

Tap water almost always needs a conditioner before use - it removes chlorine and neutralises chloramine, which (unlike plain chlorine) doesn't dissipate just by leaving water to stand. For fish that need very soft water where tap water is naturally hard, some keepers blend in reverse osmosis (RO) water to bring the hardness down.

Introducing new fish safely

Don't just tip the bag in. Float the sealed bag in your tank for 15-20 minutes to match temperature, then gradually add small amounts of your tank water to the bag over 20-30 minutes so the fish adjusts to your water chemistry. Net the fish into the tank rather than pouring the bag water in, since it may carry pathogens from the store's system.

Add new fish in small batches over time rather than stocking a whole tank at once, so your filter's bacteria colony can keep up with the added waste load.

For anything valuable or delicate, drip acclimation is safer than float-and-splash: run an airline hose from the main tank into a separate container holding the new fish, tie a loose knot (or use a control valve) to slow the flow to a few drips per second, and let it run for 30-60 minutes so the water chemistry changes gradually rather than all at once.

A separate quarantine tank, even a bare 10-20L tub with a sponge filter and heater, is the single best way to protect your existing fish - new arrivals are held and observed for 2-4 weeks before joining the main tank, catching disease or parasites before they can spread. Keep the lights off for the first evening after introducing a new fish, and watch for clamped fins, constant hiding, or refusing food for more than a day or two, all signs of stress worth monitoring.

Reading the compatibility traffic lights

Green - no known issues with your current tank.

Yellow - proceed with caution: something worth knowing about (e.g. narrow shared parameter range, a species prone to nipping) but not a dealbreaker on its own.

Red - a real conflict (incompatible water needs, predation risk, known aggression). You can still add the fish and see exactly why it's flagged, but it's rarely a good idea.

Behind the colours, the checker is looking at a few different things: whether your water parameters actually overlap, whether one fish is large enough (or predatory enough) to see another as food, and documented behaviour like fin-nipping or territorial aggression. It's a well-researched starting point, not a guarantee - individual fish have individual personalities, so a "green" pairing can occasionally still go wrong, and thoughtful aquascaping (hiding spots, sightline breaks, enough swimming space) can sometimes make a "yellow" pairing work better than the raw rating suggests.

Common beginner mistakes

• Stocking a brand-new tank at full capacity before it's cycled.
• Choosing fish before researching their adult size - that "small" fish in the store may need a much bigger tank in a year.
• Mixing fin-nippers with long-finned species.
• Skipping regular partial water changes because the water "looks clean" - clarity and water chemistry are not the same thing.
• Running an undersized filter for the tank's actual volume, rather than sizing it to the litres you actually have.
• Making large, infrequent water changes instead of smaller, more regular ones - a sudden big parameter swing is more stressful than the same change spread out.
• Only testing the water after fish already look unwell, rather than testing regularly so problems are caught early.

Filtration basics

A filter does three jobs: mechanical (trapping debris), biological (housing the bacteria that process waste - by far the most important job), and chemical (optional, e.g. activated carbon for odour/discolouration). Size your filter for your actual tank volume, and never rinse filter media under tap water - the chlorine kills the bacteria colony you've spent weeks building. Rinse it in removed tank water instead.

The three common filter types: internal/sponge filters are simple, gentle, and good for small tanks, fry, or shrimp; hang-on-back (HOB) filters sit on the rim and are easy to maintain, suiting small to medium tanks; canister filters sit externally, hold the most media, and handle larger or heavily-stocked tanks quietly. As a rough sizing rule of thumb, look for a filter rated to turn over your tank's total volume around 4-6 times per hour.

Most of your beneficial bacteria live in the filter media, not the water or substrate - which is why replacing all of your filter media at once effectively restarts your cycle. Stagger media changes (replace mechanical floss/sponge more often, but keep biological media like ceramic rings or noodles in place for months), and always rinse it in removed tank water, never tap water.