When the aquarium is new so is the seawater, but as soon as live organisms are present it begins to deteriorate. This is because of the natural life functions of the organisms, be they bacteria, shrimps, fish or corals. Fish have the most effect. So it follows that the seawater will continue to deteriorate unless something is done about it.
With a reef aquarium the inmates could well be reducing seawater content such as calcium for example, and this is easily remedied by the use of additives or a calcium reactor. However, with a fish only or reef aquarium it is not specific reduction in content that is being considered, it is the overall quality. How to describe this is difficult but the word ‘freshness’ seems to cover it. Fresh seawater is as it has mixed initially without deterioration or content reduction at all.
When the initial seawater was mixed it was probably done in the aquarium itself as it is the largest mix that will be required. There isn’t anything wrong with this provided live rock is not being used. If live rock is to be used it is better to do a smaller initial mix and make an allowance for the rock displacement – if it is a full reef construction reducing the seawater by a third usually works. If seawater is still short then a further small mix is easily done. If the rocks cause an excessive rise then the extra seawater can be siphoned off and used again.
Once livestock introduction has begun then as said the seawater quality will start to reduce. This is counteracted by routine seawater changes and the guideline amount to change is 10% of the net system gallonage. There is a reasonable argument that as the livestock is low to begin with (stocking is done slowly) then a lower amount could be changed to start with, then increased as stocking increases. It may be better however to stick with the 10% as it is a firm measure to use. As time passes the seawater quality deterioration rate will be better understood, the aquarist will have more general experience and the amount of the routine seawater change could be altered up or down.
Back to the point though, how long should the seawater be allowed to mix? The seawater needs time to reach the correct temperature, time to ensure all the salt is fully mixed and also to ensure the oxygen content is as high as possible.
The initial mix, particularly in a large aquarium but applicable to all aquariums, is the largest mix that is going to be done. It is necessary to allow more time for the correct temperature to be reached and remain stable and for the salt, of which there will be quite a lot, to fully mix. If the aquarium design is good then the oxygen will look after itself. To allow this large mix to be ready, 3 to 7 days should suffice. The lights need not be on (it doesn’t hurt if they are), but circulation pumps, the sump return pump if any, and the heater(s) should be on and left on.
With routine seawater changes the mix is going to be a lot less. There are different opinions on the mixing time needed – some say the same as the initial mix, others one or two days. I mix mine for one day normally, a day not being daylight time but 24 hours. Sometimes it is more.
The point in allowing time for the mix is again difficult to describe. This time the word best used is ‘raw’. The dry sea salt from a packet is placed into a container and tap water, or better reverse osmosis (RO) water is added. The mix is stirred by a powerhead or air pump. All the new seawater is raw, that is unexposed to natural processes. Another way of describing it – the seawater hasn’t aged.
As a boy I used to keep goldfish in a small aquarium. Water changes had to be done and I took this straight from the tap and put it in a small old galvanized bath. It was then left outside come rain or shine for a week before being used. This got rid of the chlorine (there weren’t any additives then) and allowed the water to have some exposure to the elements – that is, age.
The guideline for the maximum amount of seawater to change at any one time is 25%. Many corals and other livestock are not so happy with ‘raw’ seawater, and therefore the amount is restricted so that the unchanged seawater has the major effect.
Failure to change seawater partially and regularly usually means a slow deterioration in its quality, or freshness. Changes dilute any unwanted seawater content such as nitrate and phosphate. Changes at least partly renew trace elements and calcium etc, but it is more than that. It refreshes it – still an inadequate description but the best I can do. The wild reefs are washed by the vast seas and oceans – our miniscule seawater gallonage needs help.
So whether the aquarium is a huge one or a small nano, routine seawater changes are of great benefit. Doing them is an easy and important part of ongoing maintenance.