How much water should I change in a reef tank?

The oceans are unrivalled in their size and stability, and the corals we keep have evolved to that very, clean, pure, stable environment. In a closed aquarium we may start off with good saltwater but it quickly depletes and degrades its chemical values. Pollutants build up too, so we need to change the water.

Water changes are vital to the success of most reef aquariums. They dilute nitrates and phosphates, flush away metabolites and hormones, and if carried out in conjunction with a filter sock clean or a gravel vacuum, they take solid waste with them too. 

But the water you replace brings vital trace elements and buffers and helps to reset the captive marine environment so that it’s more like the tropical oceans again. Clean, pristine, low in nutrients, but high in elements. 

So as long as the temperature and salinity of the water change water is spot on, there is no limit to how often you change the water, and how much you change each time. In fact, regular water changes will only benefit your reef tank and the more you do them, the lower phosphates and nitrates will be, and the more constant KH, Calcium and Magnesium will be. And stability is key. 

Change the water every day and you wouldn’t even need supplemental chemical dosing, and some public aquaria with access to natural seawater nearby do exactly that – constantly replacing water in the display tank with fresh seawater. 

But the average home reefkeeper doesn’t have the time or the budget to change water every day, so there are two ways to do it. The first way is to change 10% of the water weekly. This will be enough to replace trace elements in most reefs and will keep most parameters in check. But it won’t be enough to buffer KH, Calcium and Magnesium in reefs with growing corals and additional buffering will still need to be carried out. 

The second method is to use test kits to monitor the water and to give you an indication of when to change the water, and how much. Nitrate is a byproduct of biological filtration and needs to be kept below 10ppm. Test nitrate regularly and work out how many water changes, and how much each time you need to do to keep the nitrate level in the tank at or below 10ppm.

If the nitrate level continues to climb, you need to change more water. If the level remains constant you are changing enough, and if the level steadily declines, you are changing more than you need to. But as stated previously, all water changes are good, and you can’t really change too much. As long as the saltwater you are putting in is made up from a reef recipe salt mix, and the salinity, or specific gravity, is at 1.025s.g. Test this using an accurate refractometer.

What is Triton Method?

Triton Method uses accurate analysis to monitor some 30 water chemistry parameters and tell if they are too high, too low, or just right. With Triton, instead of changing water to remedy a broad spectrum of issues they target specific elements and offer those individually to be dosed and topped back up to the correct level without having to do a water change. So some Triton Method reef aquariums go years without changing water because the water is “changed” one element at a time, and if other levels are ok, there is no point in literally flushing them away, just to pay to replace them. 

But as a general rule, change at least 10% of your reef aquarium’s total volume per week to help keep everything in check.  

Installing fish tank LED lighting
Regular partial water changes help keep parameters in balance


Posted by on

Jeremy Gay is an author, lifelong fishkeeper, and aquatic specialist. He's a former editor of Practical Fishkeeping Magazine, UK editor at Reefbuilders, a former aquatic store manager, and has collected fish in Sri Lanka and the Amazon. He's been on tv and radio, contributed to Koi Carp and Gardeners World magazines, been a product tester, a judge, and a product developer. Jeremy is here to guide and advise you on all things tropical, pond and marine, from set-up to stocking, health, feeding to breeding, as well as solving many common fishkeeping problems along the way.

Add your comment

* Required fields