Nothing is more important in marine fishkeeping than attention to water quality. Unless it is kept within very narrow tolerances at all times, success is doomed to failure. Fortunately it is not rocket technology and a few simple tests carried out on a regular basis will keep you in touch with what's occurring.

As already mentioned the basic tests for Ammonia, Nitrite and Nitrate will be familiar, as will that for testing the S.G of the water.
A further test, this time for pH will reveal just how much 'life' there is left in the water. One sign of deteriorating water conditions is the lowering of the pH value (see More About pH). Natural sea water has a pH of around 8.3 but this is likely to fall steadily in the aquarium; as it falls towards pH 8.0, the time has come for a partial water change: perhaps 10% every two weeks or 20% each month is usual.
As water is removed from the aquarium and discarded, naturally any wastes and nitrates in it are also discarded; partial water changes therefore also help to keep nitrate levels at a constant low level.

The easiest way to do water changes is to have two identical containers alongside the tank: in one, prepare the next amount of seawater, add a thermostat/heater unit, switch it on, and add aeration for 24 hours. The next day, siphon out the same amount of water from the aquarium (using a gravel washer is recommended) into the second container and discard (or use some for hatching brine shrimp!). Put the new mixed saltwater into the aquarium.

Also at this time add any regular doses of replacement trace elements. Additionally, you should remove a proportion of filter sponge medium from filters, rinse in seawater and replace; don't rinse all of the medium at any one time otherwise the bacterial colony will be adversely affected.

The protein skimmer collection cup should be emptied at regular intervals. If possible clean the protein skimmer interior surfaces to remove any fatty substances or algae, as these will affect skimming performance if not removed.

As with all aquariums, a certain amount of evaporation occurs and the water level drops. To refill the tank there is no need for you to make up fresh sea water - only 'freshwater' has evaporated, the salts remain in solution, with the result that the SG has actually risen from its original value when first mixed. Only use freshwater to top up evaporation losses.