Go marine.

The ten questions

1. Can I convert my freshwater tank?
2. What extra equipment do I need?
3. RO/DI vs tap + salt
4. How is a marine cycle different?
5. FOWLR vs reef
6. Monthly cost vs freshwater
7. Easiest first marine fish and soft corals
8. Can fresh and salt fish live together?
9. What salinity should I run?
10. Which numbers matter?

Can I convert my freshwater tank?

Sometimes the glass. Almost never the “just add salt” story. Fresh silicone, lids, and lights are often fine. The filter media, décor that leaches, and “it held tetras so it’s ready” are not. If the tank is small, scratched, or the stand is chipboard, start a marine-sized system instead of converting a kit you will outgrow. Longer note on converting →

What extra equipment do I need?

Saltwater adds: salt and a way to mix it, a heater you can trust, a hydrometer or refractometer, and usually stronger flow. Reef adds light that can actually grow coral, and often a skimmer as the tank gets bigger. You do not need a shop-full of gadgets on day one. You need stable saltwater and livestock that matches the tank.

RO/DI vs tap + salt

In a lot of Australian tap water, mixing salt straight into the tap is a gamble — phosphate, nitrate, hardness. RO/DI plus a decent salt mix is the boring, safe path. If you skip RO, test the tap first. Don’t guess. A complete system that holds numbers saves you turning the laundry into a water lab.

How is a marine cycle different?

Same bacteria story: ammonia → nitrite → nitrate. Saltwater is less forgiving and you feel mistakes faster. Traditional advice is wait weeks, then add one fish. A system built for marine from the start treats the cycle as part of the setup, not a separate science project.

FOWLR vs reef

FOWLR is fish only, live rock. Softer lighting, fewer coral rules. Reef means corals are in the brief, so light, stability, and flow matter more. Most people who say “just fish” add a coral in six months. If that is you, size and light for a simple soft-coral reef now.

Monthly cost vs freshwater

Salt, top-ups or an ATO, extra power for heat and pumps, and the odd bottle of additives. Freshwater was cheap water changes. Marine is cheap if the tank is stable; expensive if you keep crashing and replacing stock. A finished system costs more up front and less in panic buys.

Easiest first marine fish and soft corals

Fish: clownfish — a bonded pair in a proper tank — then hardy gobies or some wrasses once it is settled. Soft corals: mushrooms, zoas, leathers. Not SPS. Skip livestock that needs a specialist diet in month one.

Can fresh and salt fish live together?

No. Not in one box. Salinity is not a compromise you split down the middle. Pick freshwater or marine.

What salinity should I run?

Aim about 1.025–1.026 SG (roughly 35 ppt) for a reef or mixed tank, at the temperature you actually keep. Pick a number and stay there. Chasing 1.020 one week and 1.026 the next is how people lose stock.

Which numbers matter?

Temperature, salinity, then ammonia and nitrite (zero once running), nitrate not wild, alkalinity if you have corals. pH follows the rest. You do not need a spreadsheet. You need the same readings, same time of day.