The Fluval Sea Flex Saltwater Aquarium Kit, Black is the marine version of the 123 litre Flex — the same curved-front tank and hidden rear filtration, but with a marine-spectrum LED built for corals rather than plants.

Key Features

• 123 litres with the Flex's curved front glass and concealed rear filter compartment
• Marine-spectrum LED with app control. The spectrum is the difference: corals need blue-weighted light that freshwater plants do not
• Integrated multi-stage filtration in the rear chamber
• Programmable lighting, so a dawn-to-dusk cycle can be set and left
• Made by Fluval, the aquatics brand of Rolf C. Hagen, which has been building aquarium filters since 1975

Before You Start

• Marine is a different hobby from freshwater, not a harder version of it. Budget for a refractometer, RO water or a source of it, salt mix, and a much longer cycling period
• Nothing goes in for at least a month. Live rock needs to mature and the tank needs to cycle fully
• A protein skimmer, a return pump and additional flow are usually added to a tank this size. They are not included
• Stable is better than perfect. Salinity, temperature and alkalinity all matter more for their steadiness than their exact value

Setting It Up

• Put it where it will live before you add water. A full aquarium cannot be moved, and glass on an uneven surface cracks
• Check what the furniture underneath can take. Water weighs a kilo a litre before you add glass, gravel and rock
• A new tank has no bacteria in it. Cycling — running the filter for several weeks until it processes ammonia and nitrite to zero — has to happen before fish go in, or they sit in their own waste while the bacteria catch up
• Plant it and let it settle for a few weeks before the first fish. A tank that has been running a month is a far kinder place to arrive in
• Add fish a few at a time, not all at once

Living With It

• Top up evaporation with fresh RO water, not salt water. Salt does not evaporate, so topping up with salt water raises salinity
• Test salinity weekly with a refractometer
• Never wash biological media in tap water and never replace it all at once. Chlorine kills the bacteria living on it, and changing everything together restarts the cycle
• Unplug anything electrical before putting your hands in the tank

Good To Know

• A 123 L reef is a realistic first marine tank, but it is a serious commitment in time and running cost
• No heater, skimmer or powerhead is included. Budget for all three
• Fish-only marine is much more forgiving than a reef. If this is your first saltwater tank, start there
• A liquid test kit for ammonia, nitrite and nitrate tells you more about a tank than anything you can see