With the stock tanks chosen, the next job was to get them in place, cleaned and ready for saltwater, and to work out how to make salt water in the quantities a coral farm needs. Six large tanks hold hundreds of gallons, and every water change, top-up and new system needs more. This page covers how the Tuff Stuff tanks behaved once they were in the room, the features that worked well and those that did not, the second thoughts about leaving out the sumps, and how to mix reef-quality salt water at farm scale.

How did the Tuff Stuff tanks compare?

Right away the surface area improved a bit. These tanks are tapered rather than stepped, which gives more gallons for the same floor space. They also have a lower profile than the Rubbermaid tanks, which helps a lot in a small room. The Rubbermaids did have two advantages: strength and material.

The walls of the Tuff Stuff tanks bow out as they begin to fill, while the rigid Rubbermaids do not change shape. In this particular build that was not really a serious problem, though it may be a long-term concern. Second, the Rubbermaids seemed to be made from a less permeable material. The Tuff Stuff tanks were covered in a black residue that took significant time to clean off with water. The manufacturer does state that they are safe for use as stock tanks and do not leach contaminants into the water.

In the photos from this stage you can see two of the 110-gallon tanks along the wall and, on the right, one of the tanks filled. Looking closely, you can see the tank wall is much straighter once the vessel is full. Again, the bowing did not seem to have any negative effect.

Did the drain plugs matter?

Both brands have a drain plug. That would have been essential had sumps been built into each system, but once the sumps were eliminated it made little difference. In fact the plug sits so low in the tank that it is of little value in stand-alone tanks, because the plan was to install deep sand beds in each system, which would bury it. On the Tuff Stuff tanks the drain plug hardware is not as nice as on the Rubbermaids, and it is included only on tanks of 140 gallons and larger; none of the smaller tanks had one.

Was dropping the sumps a mistake?

Looking back once all the tanks were in, the Tuff Stuff tanks looked like they would have been the better choice right from the beginning, and perhaps another design for the sumps could have been worked out. Most of the photos of this stage were taken in September and October 2002. By February 2003 the question was still open whether there was some feasible way to add sumps to the systems. Any workable design would have meant tearing down all the tanks and giving up the plans for the display tank. In the long run, the health of the systems and the reduced maintenance looked worth that sacrifice.

Why does salt water need to be mixed carefully?

Natural seawater contains dozens of dissolved elements in fairly constant proportions. Sodium and chloride make up most of it, followed by sulfate, magnesium, calcium and potassium, plus bicarbonate and carbonate, which give seawater its buffering capacity, and many trace elements. Synthetic sea salts are formulated to recreate these proportions when mixed with pure water. For corals, the most important parameters are salinity, calcium, alkalinity and magnesium, because stony corals draw on calcium and carbonate to build their skeletons.

Salt mixes are not all the same. Brands differ in their levels of calcium, alkalinity and magnesium, and some are designed to have elevated levels for heavily stocked reef tanks. In a coral farm, consistency matters as much as the exact formula: switching between brands or mixing carelessly can cause swings in water chemistry that stress corals. Most farmers choose a salt they trust and stay with it.

What water should salt be mixed with?

The water used for mixing should be as pure as possible. Tap water can contain chlorine or chloramine, which are toxic to marine life, and may also contain nitrate, phosphate, silicate, copper and other substances that feed nuisance algae or harm corals. Most reef keepers use water purified by reverse osmosis, usually followed by deionisation, known as RO/DI water, which removes nearly all dissolved substances.

An RO/DI unit produces water slowly and sends a significant amount of waste water to the drain, so a coral farm needs either a high-capacity unit or a large storage reservoir that can be filled over time. Measuring the purified water with a total dissolved solids meter shows when the filter cartridges and membranes need replacing. A reading close to zero is the goal.

How do you mix salt water in bulk?

For a farm, mixing salt in large, dedicated containers is far easier than mixing bucket by bucket. A typical approach is to fill a food-safe container with RO/DI water, add a heater and a pump or powerhead to circulate the water, and bring it up to about the temperature of the tanks. Then add salt gradually, with the pump running, rather than pouring water onto a pile of salt, which can cause some components to precipitate.

Let the water circulate for several hours, often overnight or for about a day, until the salt is fully dissolved and the water is clear. Then measure the salinity and adjust by adding more salt or more fresh water. Many salt manufacturers give a rough amount of salt per gallon, but the actual amount depends on the brand and its moisture content, so measuring is essential rather than relying on a scoop count.

What salinity should a coral farm use?

Natural reef seawater has a salinity of about 35 parts per thousand, which corresponds to a specific gravity of about 1.025 to 1.026 at typical aquarium temperatures. Most reef keepers aim for that range. The exact figure matters less than keeping it stable, because corals and invertebrates are sensitive to rapid changes in salinity.

A refractometer is the most common tool for measuring salinity. It should be calibrated regularly, ideally with a calibration solution of 35 parts per thousand rather than fresh water, because calibrating at zero does not guarantee accuracy at seawater salinity. Swing-arm hydrometers are cheaper but less accurate, and electronic conductivity meters are accurate but more expensive. In a farm with several tanks, a single, well-calibrated instrument used for every tank keeps readings consistent.

Why does evaporation change salinity?

Only water evaporates; the salt stays behind. As tanks lose water to the air, their salinity rises. Topping up evaporated water with fresh RO/DI water, not with salt water, keeps salinity steady. In a coral farm, where open stock tanks, fans and heat drive evaporation, many gallons can be lost each day across all the tanks, so top-up is a constant job.

Automatic top-off systems, which use a float switch or optical sensor to add fresh water from a reservoir as the level drops, make this much easier and keep salinity far more stable than topping up by hand once a day. They need safeguards, such as a second switch or a limited reservoir, so that a stuck sensor cannot flood a tank with fresh water.

How often should a farm change water?

Water changes remove dissolved waste, replace elements the corals consume and dilute anything that builds up. Many reef keepers change around 10 to 20 percent of their water every week or two, though practices vary widely. In a coral farm with many corals and few fish, the demand for calcium and alkalinity may be met partly by dosing between water changes. The important thing is to change water regularly with well-mixed, matched salt water at the same temperature and salinity as the tank.

Large water changes in a farm are easier with pumps and hoses than with buckets. Having the mixing station close to the tanks, and a way to drain old water to a floor drain or outside, saves a great deal of carrying. In this room, which had no floor drain, that meant planning the route for every gallon in and out.

What should you test before using new salt water?

Before new salt water goes into a tank, it is worth checking salinity, temperature and, at least occasionally, alkalinity, calcium and magnesium. A new batch of salt or a new brand can differ from the last, and knowing the values helps you predict how a water change will shift the tank’s chemistry. Testing the RO/DI water itself from time to time, for total dissolved solids, ensures that the purification system is still working.

Freshly mixed salt water can also be slightly cloudy or have a raised pH for a while. Allowing it to circulate and aerate for a day lets it settle and stabilise. Mixing a batch a day or two ahead of a planned water change is a simple routine that avoids rushing.

Which parameters matter most for corals?

Beyond salinity, three parameters dominate the care of stony corals. Calcium in natural seawater is a little over 400 parts per million, and reef keepers commonly keep it between about 400 and 450. Alkalinity, a measure of the water’s carbonate and bicarbonate buffering, is roughly 6.6–7 degrees of carbonate hardness (dKH) in natural surface seawater, depending on location (a satellite-based global average of about 2,299 micromoles per kilogram is about 6.6 dKH, and Florida reef water near Miami has measured about 7), and many reef tanks are run somewhere between about 7 and 11 dKH. Magnesium is about 1,300 parts per million in seawater and helps keep calcium and alkalinity in balance by slowing their precipitation.

Corals and coralline algae remove calcium and alkalinity as they grow, so in a farm full of growing stony corals these values fall between water changes. Testing them regularly, at least weekly in a busy system, shows how fast they are being used. When water changes alone cannot keep up, reef keepers add them back with two-part supplements, kalkwasser or calcium reactors. Stable values matter more than hitting a precise number, because sudden changes, especially in alkalinity, can stress corals.

How much salt does a coral farm use?

More than most people expect. A farm with several hundred gallons of water that changes 10 percent a week needs to mix dozens of gallons every week, and filling a new system takes far more. Salt is usually sold in buckets or boxes sized to make a stated number of gallons, and buying in the largest practical size reduces cost. Keeping a reserve on hand means a spill, a failed tank or an emergency water change never has to wait for an order to arrive.

Opened salt should be kept sealed, because it absorbs moisture from the humid air of a coral room and cakes, which makes it harder to measure and mix evenly. Store buckets off the floor, away from the tanks and out of the splash zone. Labelling each mixed batch with the date and measured salinity is a small habit that prevents mistakes.

Why match temperature before a water change?

Water straight from a cold reservoir or a cold garage can be many degrees cooler than the tank. Adding a large volume of cold water lowers the tank temperature suddenly, which stresses corals and fish and can trigger disease outbreaks in fish. Heating the new water to within a degree or so of the tank, and checking it with the same thermometer used for the tanks, avoids that shock. The same applies to pH and salinity: the closer the new water matches the old, the less the animals notice the change.

What does salt water do to equipment?

Salt water is corrosive to most metals, and salt creep, the crust left behind where it splashes and evaporates, finds its way into everything. In the mixing area, use plastic containers and fittings, keep electrical equipment above and away from the water, and plug pumps and heaters into GFI-protected outlets with drip loops. Bags of salt should be kept sealed and off the floor, because salt absorbs moisture from humid air and cakes.

What came next?

With the tanks in place, cleaned and tested with water, the next job was to give each system a bottom: a deep sand bed that would help handle nutrients and provide a home for the small creatures that feed the corals and fish. That stage, and how the sand was seeded, is covered on the next page in the series.