Step By Step Seawater Change

Routine seawater changes are recommended for good reasons, for example minimising nitrate, re-introducing trace elements removed by protein skimming and/or activated carbon, and general freshening up.

The guideline for the amount of seawater to change is 10% of the total system net gallonage. This amount can vary once the aquarist has experience of the system, but routine changes should be done.

So, the seawater change and its preparation then. It is assumed that the aquarist has a seawater safe bucket, dry sea salt, scales, tubes for removing and replacing the seawater (tubes with a diameter of around ¾” (circa 19mm) are best), a heater, and an air pump or powerhead. The heater, air pump or powerhead do not need to be powerful, they are not dealing with a large amount of seawater. There is also a need for a hydrometer.

It is also assumed that the aquarist is aware of the amount in gallons that represents 10% of the system net gallonage.

The best fresh water for use in mixing seawater is reverse osmosis (R/O) and the aquarist will need to ensure that there is sufficient for the mixing process.

First, have a look at the instructions on the sea salt package. It is often stated how much salt is required for a particular gallonage at a particular specific gravity (SG) and temperature.

Routine seawater mixes are going to be made many times in the future so on the first occasion a little extra work is required, which will save time on following mixes.

Using the information from the package, pour an amount of salt on to the scales. Keep it below the suggested amount. Note the amount in a notebook (a notebook is a really good thing to have). Now, pour in sufficient fresh water to reach the required gallonage mark on the bucket. Give the mix an initial stir with a wooden spoon or stick.

Place the heater and powerhead (airstone) into the bucket and plug them in. Put the lid on the bucket and run it for around 8 hours or so (do not seal the lid, drill an air hole if necessary), to ensure the salt is mixed and the temperature is the same as that in the aquarium.

After the requisite period has passed, measure the SG of the seawater. Is it low? If so, add a little more salt, but weigh it on the scales first. Note the weight of the salt with the previous note. Continue with this until the SG is as desired, giving plenty of time for the salt to dissolve, two hours or so. When the required result is achieved, add up the total weight of all the salt used. This is the amount you will need on future occasions. Note the total weight down.

If after the initial period the SG is too high, add small amounts of fresh water until it is correct. When a new mix is to be prepared, reduce the amount of salt used and proceed as above, always keeping a note on salt weight for future use. It is unlikely that the SG will be too high – this is why the salt was reduced from the amount recommended by the manufacturer.

Check the seawater temperature is at the level required. If it isn’t, then the thermostat can be adjusted. Remember that SG is affected by temperature, so check the SG after any temperature adjustment.

The SG and temperature should be the same as those of the aquarium seawater.

Once the process to determine the amount of salt needed per mix is concluded, and the heater brings the temperature to the correct level, future mixes are straightforward.

The appropriate amount of seawater in the aquarium needs to be removed. This is easily achieved with a bucket (separate from the new mix bucket) and a hose. The hose needs to be long enough to reach well into the bucket when the other end is at the bottom of the aquarium.

When seawater is being removed it is a good opportunity to siphon out any debris that is seen. To make this easier by better control of the tube, the aquarist can bind the end of the tube to a length of wood which is stiff but not too wide. The wood can be around 12 to 18″ (circa 305mm to 457mm) long, depending on the depth of the aquarium and the aquarist’s needs.

Place the tube with the wood into the aquarium, then give a good suck on the other end and place that end in the bucket. Seawater will flow through the tube into the bucket. A little practice makes the ’suck’ period easy.

If any debris is noticed it is easily removed so long as the seawater is flowing.

Watch the amount of seawater that is removed and when it has reached the relevant mark on the bucket, remove the wood end from the aquarium to stop the flow. This seawater can be discarded.

Putting the new seawater into the aquarium is really easy. If the aquarium is near some stairs, put the new seawater bucket on the stairs and trail a tube down to the aquarium. It may be necessary to have a different tube for this. It is best if someone can ensure that the tube does not come out of the bucket. At the aquarium end, another one of those technical ’sucks’ will start the seawater flow into the aquarium, Once the bucket is empty the flow will of course cease.

If the aquarium is not near stairs, no matter. The aquarist can use a powerhead (use the one in the new mix) and a suitable diameter tube, and pump the seawater in. The powerhead will not remove all of the seawater, the last bit can be put in with a small jug.

All that remains is to check the SG of the aquarium seawater (with the new seawater in) to ensure all is well. If the SG is a little low, just increase the amount of salt a little on the next new seawater mix. Likewise, if it is a little high decrease the amount.

All that remains is to dry the new seawater bucket and store it until the next time, along with its heater and powerhead or air pump.

One note – when removing the heater from a new seawater mix be sure the heater is cool. If it is not and is out of the water, then the glass could crack.

For the first two or three new seawater mixes check that the SG is as desired, and do an occasional check after this. SG checks of the aquarium seawater itself should be done routinely

Routine seawater changes are simple and quite quick if the above suggestions are followed. Anything that simplifies routine maintenance and gives more time for watching the aquarium can’t be bad.


Step By Step Seawater Change
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