Seawater Changing – Always The Same?


As has been repeated so many times before, seawater quality is the number one requirement for a successful marine aquarium. Having said that, is there any variation on the change regime?

For a new marine aqurium the guideline amount to change is 10% of the net gallonage weekly. A new aquarium needs time to settle down, or to put it a better way, to stabilize. This period can vary and during this time there is a requirement to carefully monitor conditions by testing.

Marine systems don’t follow one format, they can have different equipment fitted and different inhabitants. This is the heart of the matter really, the type of aquarium.

Let’s go back a bit though. As said, there is an initial guideline of 10% for seawater changing. This should be adhered to in the early days of the aquarium and at the same time seawater quality testing should occur. This testing at its base should include specific gravity (SG), temperature, ammonia, nitrite, nitrate and pH. What is being watched for is reasonable stability and acceptable levels of toxics. Once the system is initially mature, that is ammonia and nitrite have disappeared, nitrate will usually make an appearance as the end product of ammonia/nitrite conversion (this doesn’t always occur but that will not be gone into here). Nitrate needs to be kept low in any system, but the guidelines are for a reef less than 10 ppm (parts per million) and fish only less than 30ppm, though the latter should be as low as is possible as well.

Seawater changing should continue until there is an indication of the rate of rise in nitrate. It is very good practice to jot down in a notebook the nitrate level at each test, in this way eventually the rate of rise can be predicted.

Once the aquarist has knowledge of the nitrate trends then consideration can be given to seawater changes. If the nitrate contiunues to rise but more slowly, then an increase in the amount of the seawater change can be considered (and also a check made of the feeding regime, to avoid overfeeding). If the nitrate has not increased but is stable at an acceptable level, then the change amount is seemingly adequate. If there isn’t a nitrate presence, then it is possible to reduce the seawater change amount. However, in this case take into consideration the fact that a change not only controls the nitrate, which is used as an indicator, but also replenishes to a fair extent trace elements, and others such as calcium. It could be best to continue at the guideline amount.

There are mainly three kinds of saltwater aquarium systems: the fish only system, the mixed reef (fish and corals) system, and the corals only system. At a guess the most popular is probably the mixed reef, followed by fish only then corals only. Should the seawater change be the same for all three?

It is generally thought that the heaviest nitrate probability is with the fish only system. This is because these have the greatest number and/or size of fish present, and fish require feeding more. It follows that the mixed reef is the next in line for nitrate, followed by the corals only system.

The same guideline applies to them all, 10%. It is likely that this will be excessive or adequate for a coral only system, with that possibility reducing for a mixed reef and much reduced for a fish only. Again, the same action can be taken in response to ongoing test results.

So it is clear that there isn’t a rule for seawater changing. The guideline of 10% is for the early months as the aquarium matures and trends are discovered. Following this, the aquarist can make a reasonable determination on ongoing action.

Routine seawater changing should not be abandoned or over extended even though tests indicate it could be. Seawater changing is of great benefit to the health and vitality of the inhabitants. There may be equipment fitted such as a protein skimmer, but such equipment is an aid to seawater quality and not a guarantee of it.

  1. Thanks for this. I have read so many variations in water changes but felt my three month old reef+fish tank (75 gallons) does the best with 10% weekly changes. I’ve gotten picked on by my boyfriend who thinks it should be every other. Now I have some evidence to back up my weekly changes!

  2. I like the point about using water changes to introduce trace elements consistently – this is something that I hadn’t considered when I first had my saltwater aquarium. It almost seems to me that in a well balanced system (if you don’t have a lot problems with waste and nitrates) that this is the more important reason to change water consistently – but that’s just my feeling at the moment

  3. Hi Rachel.

    I wonder what your boyfriend’s reasons for his opinion are? Maybe your water tests show high quality and he thinks you’re overdoing it! I like your website.

    Hi Luke.

    Yes, routine water changes are of great help with trace elements, it at least partially replaces them. It needs to be remembered though that other constituents such as calcium may be used up more rapidly by for example hard corals, so testing is the way to ensure that there is sufficient present.

  4. Saltwater aquariums are good choices in setting up tanks for the fish hobbyists. However, home freshwater aquarium can also be a great source for raising your favorite fishes.

  5. I have 2 angels in my 55 gallon tank, one is quite large and they are both fine, i have a variety of different fish and they all get along quite well, if you are worried about getting angels, stat small and moniter them closely.

  6. Assuming the angels are of the marine variety and not a Centropyge (dwarf) species it should be remembered that they will grow! Many are beauitiful and very tempting when small but as they grow they change colour to become equally lovely. They can grow to 12″ (circa 30.5cm) or so and therefore need a large aquarium.

  7. I honestly did not have an idea on routine water changes when it comes to seawater on marine aquariums. Thanks for this information though. It gave several variations on water changing.

  8. Hello Toby.
    Glad as always that the text was useful.

Comments are closed.