Cure Nuisance Algae

Q) My marine aquarium has been plagued by slime and hair algae almost from the day I started stocking up. What steps can I take to get rid of it? It spoils the whole appearance of the showtank.
C. Crawford, Washington DC, USA.

A) I have long since lost count of the number of marine aquarists who have written to me threatening to leave the hobby as they endure the scourge of nuisance algae. It is possibly the only area of the hobby guaranteed to affect practically all hobbyists at one time or another, with persistent and serious cases being reasonably commonplace. Both filamentous and slime algae can reduce an attractive underwater scene into a ghastly mess, leading the marinist to believe that collecting cigarette butts could be a rewarding hobby by comparison!

Having said that, all is not lost; for even the most badly affected tanks can be revived to their former glory (if they had one!). BUT it is vital to stress that the hobbyist must be HONEST about the type of arrangement he or she has, and be VIGILANT when trying to cure this menace. If your tank is doubly overstocked, fed four times a day, has no protein skimmer and rarely receives a water change, then clearly a great deal must change if nuisance algae is to be eradicated for good. Correct husbandry should always take precedence.

Understanding The Enemy
Whilst I do not propose to devote any time to the species involved, it is important to realise that ALL types of nuisance algae are encouraged to thrive in poor, or deteriorating water conditions. This generally means an abundance of nutrients in form of nitrates, phosphates, sulphates and silicates, etc., greatly in excess of those that can successfully be dealt with by the aquarium filtration system.

Excessive nutrients can arrive in the average marine aquarium by a number of common routes and each will have to be carefully addressed in turn by the hobbyist if a successful cure is to be effected.

The most common cause of nuisance algae is water changes and replacement of evaporated water with unfiltered mains TAPWATER. Tapwater is designed to be safe for humans and the keeping of fish or any other forms of aquatic life does not form part of the vocabulary with the vast majority of water companies. In general, they follow the regulations as set down by the World Health Organisation, often falling just short of the maximum permitted levels quite legally. Unfortunately, this is frequently not acceptable to the marine aquarist and arrangements must be made to filter tapwater into a usable condition. Many people find that nitrate/phosphate/sulphate removing resins are all that is required. Others, less fortunate, will have to turn to more sophisticated devices such as reverse osmosis units. I, personally live in an area where the mains water quality is fairly atrocious as far as fishkeeping is concerned and borders on the need for a passive filtration resin and an RO unit. Whilst reasonably expensive as an initial outlay, it is becoming evermore clear these days that the purity of reverse osmosis water should be seen as less of a luxury and more of a necessity if marines are to be maintained with the minimum of problems.

However, even the use of RO water cannot rid the system of unwanted algae if the correct WATER CHANGE procedures are not followed.

It is essential that water changes are of the highest quality and performed regularly if nuisance algae is to be avoided or brought under control. Most aquaria will benefit from a 15-25% high-quality water change every two weeks. Not only will this remove excessive nutrients but it will also dilute those remaining. On a regular and vigilant basis, most tanks can hope to see a noticeable improvement within 4-12 weeks if RO water is used. Performing water changes with impure water will only exacerbate the problem, as will the replacement of evaporated water with 'straight' tapwater. If you do not wish to buy a reverse osmosis unit yourself, many forward-thinking retailers now supply it; some even add the salt and heat it to provide an instant water change mix! It's well worth investigating if a busy lifestyle demands it.

Topping-up evaporated water loss can involve a considerable amount of water ‘exchange’ each week. By replacing with ordinary tapwater, the impurities introduced become progressively more and more concentrated as only pure water molecules evaporate and the unwanted substances are left behind. Therefore, always use distilled, de-ionised or reverse osmosis for top-up purposes. It is worth noting at this point that fashionable bottled mineral water is not filtered water and may contain a substantial amount of algal nutrients. Many people are surprised to learn from the contents label that bottled water is not really much better than their own water supply (except that it costs considerably more!).

Salt, Carbon, Substrate and Rocks
Activated carbon and salt mixes can be a source of undesirably high levels phosphates and nitrates, with price no guarantee of purity. Test your regular brand by mixing a litre of salt mix with distilled water to a specific gravity of 1.021 and checking with the appropriate test kit. Activated carbon can be confirmed safe by dropping a few grains into 10ml of distilled water and letting it stand for and hour or so. Remove the grains and test the water for phosphates (nitrates will hardly ever be found). In some instances even tufa rock and coral sand are worth investigating in the same way. The results of these tests can be quite revealing and are often ignored as a source of nutrients for nuisance algae.

An often asked question is whether the bubbles given off during the day by slime algae, in particular, are harmful. The answer is no. This is merely oxygen given off as a by-product of photosynthesis (although knowing that does not make it any more attractive, I have to admit!). At night, however, carbon dioxide is generated by the very same process and high levels of this gas may stress livestock.

Is there such a thing as toxic algae? Whilst algologists tell us that there certainly are species of toxic algae in the world, these are highly unlikely to find there way into the marine aquarium. Generally speaking, this sort of scenario is not worth worrying about and much rarer than some people would have us believe.

An over-abundance of waste products by livestock (mostly fish) quickly leads to a deterioration in water quality and an ideal environment in which nuisance algae can thrive and multiply at an alarming rate. It is, therefore, crucial to adhere to traditionally accepted stocking levels: as far as the FISH-ONLY tank is concerned 1" of fish for every 2 gallons nett is the very maximum to be considered safe after slowly stocking for one year. In the REEF AQUARIUM, fish stocks are far lower at 1" of fish for every 6 gallons nett. The key word here is 'nett' as these calculations can only be made after the displacement of rockwork, sand, gravel, decoration and ullage (ullage? It’s the space above the water surface to the top of the tank) have been taken into account. Such items can effectively reduce the water holding capacity of a tank by 20% or more. It is easy to see that by working on gross gallonage, the aquarium could be overstocked by at least one fifth! Equally, the extra stock would have to be fed, which ultimately means that the tank is not only overstocked but overfed as well!
Assuming that the main biological filtration is adequate, correct mechanical and chemical filters must not be overlooked. The need for a protein skimmer and activated carbon cannot be stressed too greatly. Not only do they assist greatly in the removal of substances that cause pollution but they are the only devices that can perform such an important task. Having said that, in at least 75% of cases encountered by me, the size and efficiency of the skimmer is woefully inadequate. It is not sufficient to HAVE a protein skimmer, it must be the right one for the job. An efficient skimmer can remove copious amounts of waste at an early stage, keeping the tank water clean and an inhospitable place for unwanted algae. The same applies to carbon; it must be activated (not the freshwater type), of high quality, positioned properly in the system and changed regularly. Make no mistake, the installation of a really effective skimmer and good quality carbon can improve the fortunes of an aquarium markedly.

Being so adaptable, nuisance algae can proliferate under practically any lighting scheme and it would be a mistake to start altering lighting systems drastically without first rectifying any shortcomings in the quality of water. Occasionally, altering the source of lighting only serves to increase the vigour of unwanted algae!


Two additives should be avoided at all costs: liquid invertebrate food, which can prove to be highly polluting, and any form of algal fertiliser. Most tanks will support a fine growth of macro-algae without the need for extra nitrates, etc. Algal fertiliser cannot be selective, feeding both decorative AND nuisance algae alike.

UV and Ozone

Both ultra-violet sterilisation and ozone will help to improve the marine aquarium environment as a whole and their installation can only be seen as a welcome addition. However, neither will noticeably reduce nuisance algae as such, and money put aside for such items might be better invested in a good reverse osmosis unit.


Whilst some herbivorous fish will browse to a variable extent on filamentous algae, there are few suitable aquarium inhabitants willing to consume slime algae. The Caribbean Queen Conch has been suggested as a suitable predator but its sheer bulk may cause disruption in many reef tanks; it could also be difficult to acquire outside the USA. Of course, if a potential grazer is chosen, it should be remembered that its waste will ultimately contribute to the overall condition of the water as well; in reality the benefits and disadvantages of filling the aquarium with potential predators may largely cancel each other out.


The use of chemicals can be considered only as a last resort and a sign of desperation. Experience has shown that their success rate fluctuates wildly and the effects are nearly always very temporary. Fish and invertebrates have also been observed to suffer as a direct result of such treatments. Think VERY carefully before embarking on this route!


Whether your algae be slimy, filamentous, or both, the most reliable approach has always been to keep water quality extremely high. If this route is pursued vigorously, a gradual improvement is nearly always noticed and even badly affected tanks can be transformed in only a few short months. Unfortunately, there are no easy options but identifying the possible causes to this frustrating problem is a big step on the way to solving it.

To recap on positive remedial action:

  1. Always use a mains tapwater filter; reverse osmosis if possible.
  2. Test salt and carbon for nitrates and phosphates.
  3. Perform the correct water changes, regularly.
  4. Never use 'straight' tapwater for evaporation top-ups.
  5. Be realistic about aquarium volumes. Always measure the nett amount accurately when it is first filled.
  6. Never overstock with fish.
  7. Always base stocking levels on the nett volume of water (see no. 5).
  8. Feed livestock sparingly.
  9. Don't use chemical ‘cures’, algal fertilisers or liquid foods.
  10. Always fit an appropriate protein skimmer and activated carbon.
  11. Don't add yet more fish in the vain hope that they may eat the algae.
  12. Do remove unwanted nuisance algae at every opportunity.
  13. Don't rely on decorative algae to 'rob' nuisance algae of its nutrients if the situation is still really acute (it usually gets smothered and dies!).
  14. Be honest about assessing stocking levels of fish, frequency of water changes, etc.
  15. Be vigilant. Eradicating nuisance algae will require some effort, but the rewards will be there in the end.
© Nick Dakin. May not be used in whole, or part, without permission.