Seawater Changes – The Suggested 10%

Seawater is the number one most important item in the marine aquarium. If the seawater is of poor quality then there are going to be problems, perhaps sickly fish, unexpanded corals, horrible algae and the like. Definitely not required by any aquarist.

Some problems could be caused by other things, for example in a reef aquarium it could be lighting. However, seawater has to be of high quality.

In research or given advice by a storekeeper this 10% seawater change pops up. On this website 10% of the total seawater gallonage including any sump is recommended as the amount for a weekly change. New aquarists doing things ‘by the book’ could run into problems and don’t understand why: everything is being completed correctly including the 10% partial seawater change.

So let’s look a little closer at some reasons that seawater can deteriorate remembering that we are thinking of a new aquarist.

First it’s assumed that stocking is correct. Well, it could be at the moment but two things could occur. Maybe the eventual size of the fish was overlooked and as they grew the aquarium became overstocked. Second, the aquarist is concerned, rightly so, that the fish have enough to eat. The fish sometimes have a tendency to hang around the food area even after they’ve eaten their fill and this causes the aquarist to put more food in. This concern is good but the extra food is not. Third, the aquarist is keen to keep the viewing glasses clean as they are obvious but not so much about the sand and any detritus at the bottom of the aquarium as it doesn’t look too bad. These few reasons seem to be the major cause of seawater deterioration. If it deteriorates too far and problems arise then the pleasure of the marine aquarium could be reduced or lost.

The one major process that needs to be understood is the nitrogen cycle. It’s straightforward and there isn’t any mumbo jumbo scientific stuff required. Detritus in the aquarium decomposes and as it does so it produces ammonia which is deadly to livestock. Ammonia is converted to nitrite which is also dangerous. Then nitrite is changed to nitrate which is not dangerous but in sufficient amounts can cause problems such as algae. That is why it is necessary to be sure that the nitrogen cycle has been established before livestock is introduced. This is all understood by aquarists including new ones but there are pitfalls. In a new aquarium the nitrogen cycle is not stable and time, achieved by the slow introduction of livestock, needs to be given for this even though the introductory process for the cycle has been completed. Once the livestock are fully introduced stability will be achieved after a little more time.

Anyway, 10% seawater changes! The aquarist does this weekly and all is well until, say, the nitrate is noted to be rising. Why? First, what kind of biological filter is in use? Is it live rock? If so there could be a problem with the amount though this is unlikely if proper research and or advice has been obtained. Is it a power filter? This can create a problem as these filters do not complete the full nitrogen cycle as they don’t deal with nitrate. Seawater very low in oxygen is needed to deal with nitrate and oxygen laden seawater is being pumped through the filter. What to do about nitrate?

First, check the fresh water supply, is it good enough? It is always a very good idea to use an RO (reverse osmosis) filter to run the fresh water through as this will generally  produce 95 to 98% pure water. No problem there. RO filters are not unreasonably expensive.

The power filter, is it regularly maintained? The biological part should not be generally touched but the anti-detritus part should be cleaned regularly.

Next, the aquarist needs to self check! First and foremost, feeding. Don’t overfeed. Once the fish lose the desire to grab every morsel, stop even though they continue to hang about. Experience will tell, as experience builds exercise caution. Is the aquarium maintained properly. When a partial seawater change is in progress siphon detritus out with the seawater, it’s very easy. Poke the siphon tube into every accessible area where detritus accumulates. If the siphoning doesn’t cover all necessary areas in one go, do the other half the next time.

It’s assumed that any livestock overload because of size or numbers has been dealt with. If not, it needs to be done.

The 10% seawater change per week is a guideline not a rule. Importantly, it’s the initial guideline. Ammonia, nitrite and nitrate tests should be completed and the first two should be nil or no trace. If nitrate is present then take a note of the reading at each test. A small notebook is of great help, just jot the reading down. After a while the trend will be clear. If the reading is increasing, check all the suggested items already  mentioned. Increase the percentage partial seawater change and continue to monitor. A point will be reached where the increase stops and after, if maintenance disciplines are in place, could well begin to reduce. Continue with the increased changes until the nitrate is acceptable (in fish only systems the guideline is 30 ppm (parts per million) or less, and in reef systems 10 ppm or less. Remember these are guidelines.) Then continue with 10% or the proven change percentage and continue to monitor. React as required.

As mentioned the 10% for seawater changes is a guideline. However, if the seawater condition in the aquarium is acceptable and continues to be so it is recommended that 10% weekly seawater changes continue. This is because the aquarium seawater is freshened, also any reduced or lost constituents are replaced or at least partially so.

Keeping a notebook could seem as though things are becoming too serious. Not at all. In addition to the seawater monitoring notes can be made of when corals and fish were purchased, a good place to get replacement lighting and when the replacements need to be done etc. The notebook is very useful.

Just to repeat – guidelines are just that and if necessary temporary because adjustment is required. The aquarist needs to be self critical and change procedures if they are suspect. Once experience has been gained many problems don’t occur.


Seawater Changes – The Suggested 10%
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