But when you do a water change on a saltwater tank you are removing both salt and water - so I'd need storage for salt water as well - am I correct? Do you know what percentage of water is removed by most reefers for a water change? Surely not the 70-80% that I'm used to for my freshwater tanks?
MEASURING SALINITY
You buy a hydrometer or refractometer to measure the salinity (salt level) in the marine tank.
Hydrometers come in a glass tube style and a plastic chamber style. The plastic chamber hydrometer is a little more expensive than the floating glass hydrometer, however the plastic chamber is less likely to break. The glass ones tend to break.
A refractometer is more expensive than a hydrometer and more accurate, but they aren't that expensive. However, they can take a bit to work out what you are looking at when measuring the salt level.
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TOPPING UP THE TANK
If you have coverglass on the tank, it will significantly reduce the amount of evaporation. If you need to top up the tank between water changes, you simply add a bit of freshwater to get the salinity correct.
I used to set the tanks up and use a permanent marker to make a line on the glass where the water level is. Then just add freshwater to top up to that mark.
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WATER CHANGES
When you do a water change on a marine tank, you remove minerals (including salt) that is dissolved in the water. You just gravel clean and drain it out like a fresh water tank.
To fill the tank, you make up the salt water at least 24 hours before you use it. The new water should be checked for pH, temperature and salinity.
If you have chlorine in the tap water, you can dechlorinate the fresh water before adding the marine salts. However, if you have chloramine in the tap water, you make up the salt water and aerate it for 24 hours, then add a dechlorinator about 5-30 minutes before using it in the tank.
If you are using reverse osmosis water, it should be free of chlorine/ chloramine and you theoretically shouldn't need a dechlorinator. However, it depends on the R/O unit and how old it is, as to whether it removes all chlorine/ chloramine.
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A lot of people don't do water changes on marine tanks because they have mostly corals and a few fish. They have protein skimmers, refugiums in trickle filter/ sumps that hold macro algae and that help to use nutrients. They usually add supplements like calcium on a daily or weekly basis.
I used to do 90-95% water changes on my marine tanks. I used natural sea water from the ocean and simply drained the tanks each month and refilled them. I just left enough water in the tank for the fish or shrimp. I had a lot of marine algae in the tanks and live rock. I have used protein slimmers over the years but don't like them that much because they remove most of the plankton and that includes baby fish and shrimp.
