Fish disease diagnosis

Fish disease treatments

Koi ponds, aquaria & filtration



Freshwater fish


New filter start-up

managing the water

Keep readings at an acceptable level

If the test kit readings show an elevated ammonia or nitrite level, it is important that steps are taken to reduce it and bring it back to an acceptable level. It should be pointed out that we are not aiming for a zero level, as there has to be a low level of ammonia/ nitrite to encourage the nitrifying bacteria.

Water changes

The simplest way to control ammonia / nitrite levels is to dilute the build-up of toxins by carrying out partial water changes. The most common problem is that the levels are allowed to build-up too high, which means massive water changes are needed just to bring the levels back to an acceptable level.

The aim should be little and often - usually changing between 10 -25% of the water, depending on the test kit reading. For example, if the reading is twice as high as the acceptable level, one would need to do a 50% change in order to bring the level back in line.

Stating the obvious, the higher the initial stocking level, the more water changes will be needed to maintain an acceptable level. Which of course is why fish should be gradually introduced to the new pond / tank. The other influence is feeding. Feeding should be kept fairly light during the first few weeks and certainly no more than the fish can eat within 2-3 minutes, once or twice a day.

As the filter starts to mature, the need for water changes will decrease.

Water changes

Ideally, when doing water changes the replacement water should be treated with a de-chlorinator (available from an aquatic shop),  and preferably been left to stand for 24-hours. Whilst this is easy for a tank,  this can be difficult for a large pond as the water needs to be treated with a de-chlorinator before it is put in the pond. This why it is best to try and reduce ammonia production (and subsequent nitrite production) by gradually increasing stocking and feeding levels as the filter matures.

However, if ammonia / nitrite levels do reach a dangerous level, this represents more of a risk to the fish than a water change with untreated water. Such is the danger of elevated levels of these two toxins.

Other ways to assist

Elevated ammonia and subsequent nitrite levels can be reduced by using zeolite in the filter system. This silicate rock removes ammonia from the water. See the zeolite page for more details on usage. This can be useful where there is a particular problem - i.e. stocking levels too high with elevated ammonia levels despite regular water changes.

Elevated nitrite levels can be assisted by adding salt to the water at a rate of 3g / litre. However, this only helps by blocking nitrite from entering the fish's blood and does nothing to prevent stress, skin and gill damage. It can also be difficult to keep track of the salt concentration if you are doing water changes.  Salt is very useful in assisting against raised nitrite levels - but it is important that it is used in conjunction with water changing in order to keep nitrite levels as low as possible.

Filter start-up products are available which claim to "seed" the filter with suspended bacteria to speed up the maturation process. I am not aware of any work carried out to verify these claims - so it is a question of suck it and see!


Fishdoc would like to thank the following benefactors for their sponsorship of this site: - Sho Koi - Aquadyne Bead Filtration - - Koizyme (Genesyz Aeromonas Fighter

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