faulty thermostat/heater, insufficient heater capacity.

Solution: replace/upgrade heater

PH

can be affected by numerous factors, for example Carbon Dioxide lowers it – PH drops overnight when plants are not photosynthesizing and gradually increases when the lights come back on. Water changes help in maintaining stability and buffers can be added to keep it within acceptable limits – these can affect KH so care needs to be taken.

Ammonia /Ammonium

Ammonia is highly toxic but in certain conditions forms into ammonium which is slightly less so – likely to be Caused by overstocking/feeding or the death and decomposition of livestock. Can be successfully cleared By chemical additives, diluted by water changes, but most importantly, by identifying/rectifying the cause.

the next stage of the Nitrate Cycle – less toxic than ammonia but still potentially deadly (more so in Freshwater than saltwater). Similar cures as for ammonia, but the cause needs to be identified and rectified.

the stage of the Nitrate Cycle we normally aim to achieve – these are much less toxic and tolerated by most species quite well. However, levels should be kept to a minimum as high levels are toxic and fuel nuisance algae. In freshwater, water changes help enormously, though there are numerous methods for reduction (resins, bacterial additives, various types of denitrator). In marine tanks (reef in particular), nitrates are more toxic and many species far less tolerant. Nuisance algae is fuelled and can smother/kill corals. Water changes are more expensive, so alternatives are necessary for larger tanks – denitrators (though correct flow is essential),or algae growing in the sump (have this lighting at opposite times to the main tank as this is a good way to reduce nightly swings in values). There are resins that can ‘soak up' nitrates, but the best answer is to ensure minimal nutrients in the tank (reduce stocking/feeding).

many foods and additives and even your source water can add to the phosphates in your tank. High phosphates can be toxic and whilst it is essential to life at low levels, should be kept down to where it does not register on the test kits available to most of us. Ensure you don't add it in the first place by ensuring as much as possible that products don't contain it (for example some cheap brands of activated carbon). Water changes will help with an immediate reduction, followed by the continued use of adsorption resins (in a reef tank, it is safest to use an iron based one as aluminium based ones are believed by some to affect some corals).

Salinity / Specific Gravity

whichever of these you test for, the value can increase due to evaporation – frequently top up with FRESH (RO) water to maintain stability. When mixing replacement water, ensure it is the same as in the aquarium.

levels can drop in reef tanks due to its uptake by hard corals and the growth of calcareous algae (usually attractive colouration and thus quite desirable). In small tanks, this can be boosted by adding supplements though for larger tanks, more efficient/cost-effective methods would be the use of a calcium reactor (some of these require carbon dioxide to reduce the PH allowing the media to be dissolved by the water).