Marine aquarists aim to maintain seawater which is of the highest quality. The reward for doing so is a vibrant aquarium, be it a fish only, corals only or mixed reef, and without the intrusion of nuisance algae.
‘Quality’ means the seawater is stable, the pH in the region 8.0 to 8.4, nitrates in a fish only at 30 ppm (parts per million) or less, in a reef system 10 ppm or less, and phosphate preferably undetectable. If the aquarium contains a captive reef the aquarist will quite likely maintain alkalinity, also calcium if this is relevant to the livestock. However, for the purposes of this discussion quality means stable pH and ‘proper’ nitrate and phosphate levels, that is, they are within the guidelines.
Aquarists complete routine seawater changes which go a long way towards maintaining high quality. Any undesirables in the seawater are diluted and, at the same time, trace elements are replenished or partially so. The seawater could be said to be freshened. The initial guideline amount for routine changes is 10% of the total net gallonage of the system weekly. This therefore includes any sump. There isn’t any requirement to change 10% every week; the aquarist could find it more convenient to change 20% fortnightly. However, a weekly change probably contributes to stability in itself, as the change caused by raw seawater being added is smaller.
pH as said could be anything between 8.0 and 8.4. Whichever the measurement is it should be reasonably stable. It is possible, in some circumstances probable, that there will be a fluctuation in pH between the lights on and off periods. This is normal as it has to do with seawater dissolved gas levels. If there is an excessive reduction, or any reduction which worries the aquarist, one way of combating the change is to employ algae in a sump. The macro algae Caulerpa is commonly used and is lit by fluorescent tubes which have an opposite cycle to the display aquarium lights, which means that when the display aquarium is lit the sump is not and vice versa. Plant growth tubes are often used to assist the Caulerpa, these tubes have a lower Kelvin rating than lights commonly used on a display aquarium. The Caulerpa will also act as a natural filter to combat nitrate and phosphate.
Sometimes pH is not varying excessively, it is just too low. If the reading is consistently too low, and by this it is meant below 8.0, then it is possible that the system does not have efficient enough gas exchange. Gas exchange occurs at air/seawater interfaces, particularly at the aquarium surface. It also occurs on protein skimmer outlets, weirs and the like. It is at these interfaces that the seawater takes in oxygen which is clearly important for livestock. Too low an oxygen level could have serious repercussions for the livestock under certain circumstances, and it could also be the cause of a low pH reading.
Testing the seawater for sufficient oxygen relevant to pH is very easy. Measure the pH of the seawater in the aquarium and make a note of the result. Siphon a gallon or two of seawater into a safe bucket, the one used for new seawater will do. Importantly maintain the temperature so it is the same as the aquarium and also circulate the seawater vigorously for two hours or so. The heater used for new seawater will be fine, also the powerhead or air pump. Note that the seawater needs to be moved vigorously, if it is sluggish it will not do.
After the required time measure the pH of the seawater in the bucket. If it is the same as the aquarium then seawater movement in the aquarium should be adequate. If the pH is higher than that in the aquarium then increasing seawater movement in the aquarium should be beneficial. Sometimes an additional powerhead will do. Place it about half way down in a convenient and hopefully hidden from view spot, and point the outlet at the surface or at an angle across it. Seawater will be moved either in a mound at the surface or a stronger flow will move across the surface. In either case gas exchange should increase affecting the pH. Before an additional powerhead is used it could be worthwhile checking the positions of existing devices.
If the pH is too low and the above test doesn’t make any difference, maybe the alkalinity is also too low. Keeping it brief and basic, seawater is on the alkaline side of the pH scale. Life actions of livestock continually try to push the pH towards the acidic side, particularly in heavily stocked fish only systems. Increasing the alkalinity could maintain pH at a more desirable level and is worth trying. If alkalinity is adjusted it is usual to keep it above natural seawater levels. If it is intended to try this, then there isn’t any mystery or need to explain measurements. Test kits and adjustment powders are commercially produced and easily obtained. The instructions will explain all and there will be a measurement chart included.
The aquarist could be having trouble with nitrate and/or phosphate. If routine seawater changes are properly done and excessive readings persist there are checks that should be initially considered.
First is feeding as this is the major culprit in many seawater quality problems particularly with novices. Nitrate is a product of the nitrogen cycle and is the end product with a canister filter. Even the acclaimed live rock cannot deal with some nitrate readings. Phosphate is mainly introduced to the seawater by feeding. Both nitrate and phosphate have been implicated in problems with nuisance algae, so excessive readings need to be reduced. There is information on feeding techniques in the Articles section on aquaristsonline.com, so further explanation will not be given here. The tendency to overfeed usually has two causes, the first being the pleasure of feeding and the second the concern that the livestock get enough. Both are good reasons, but the act of overfeeding will do harm. As said, the first action is to carefully check the method of feeding and the amount fed.
Nevertheless, some aquarists have trouble anyway, despite being disciplined when feeding and despite doing regular seawater changes. It has already been stated that the initial guideline for routine seawater changes is 10% of the total system net gallonage. Note the word ‘initial’. It may be that the amount changed needs to be increased. It could very well depend on the livestock load. A fully and correctly stocked mixed reef system will usually have a lower bio-load than an equivalent sized fully and correctly stocked fish only system. Fish present a heavier bio-load than corals. So for example in a fish only system there is heavier feeding because of the higher number of fish and of course the fishes life cycles continue. Thus there is more scope to see excessive nitrate and/or phosphate readings. The answer here is probably dilution. In other words an increase in the amount of seawater changed routinely. If the aquarist is changing 10% this could be increased to 15%. Have the readings begun to subside? The readings will gradually subside, as additional pollution continues to be produced – it will not suddenly or quickly disappear. If not, try a further increase to 20%. It is not normally recommended to change more than 25%.
It has to be reiterated that increases in the gallonage of routine seawater changes come after the aquarist is fully satisfied that other avenues to solve the problem are exhausted, such as the mentioned feeding. It is also important to examine carefully the filtration which in the first instance should be adequate. Filtration to combat nitrate and phosphate could be increased. In the case of nitrate more live rock for example, and/or if there is a sump the introduction of a DSB (deep sand bed) and perhaps some Caulerpa algae. In the case of phosphate perhaps an anti-phosphate resin filter could be considered.