How To Tell When Your Aquarium Has Cycled

A great deal of fun and excitement is caused by the planning and building of a new marine aquarium. All the equipment to consider and choose, and the controlled excitement of when the first live stock will appear.

Provided the aquarist has chosen and set up the equipment with care, then there isn’t any reason why the fish only aquarium or reef aquarium should fail. Except one!

If the aquarist has researched the following will be known, but it is surprising how many problems arise because of missed or inadequate preparation in this area. The worse scenario is the death of livestock that have been so eagerly anticipated. Yet if this final step had been completed properly the story would have been very different.

The final step is the maturation of the biological filter. This is the filtration that completes the [tag-self]nitrogen cycle[/tag-self], or some of it. Toxics that are lethal are removed (ammonia and nitrite) by the aquarist’s good and hardworking friends the bacteria. These bacteria do not just appear however, they need to be introduced and develop in most cases.

For example if the aquarist has decided to use a trickle or canister filter then the necessary bacteria will be totally missing. Put fish in now and..well, I’ve already mentioned it.

I’m against using hardy fish to start off the filtration, both on ethical grounds and because these fish can be a problem later – they are often aggressive and quarrelsome. So I like the use of a commercial fluid, easily obtainable, to kick start the filter. Full instructions are given and should be followed. The bacteria will appear and develop, first dealing with ammonia and then nitrite. The aquarist needs to monitor the levels with a test kit.

What of [tag-self]live rock[/tag-self]? Live rock comes with bacteria, ones that can deal with ammonia and nitrite, and also (unlike canister and trickle filters) nitrate within reason. So no problem then? Hopefully not, but it must be remembered that live rock has gone through processes, often at the retailers, before it is sold. Much of the life on the rock will have died, and it is possible that the bacteria populations may have been reduced too. This raises the possibility that they cannot deal with a higher bio load straightaway.

Live rock when it is being ‘cured’ often sits in water that has a lot of ammonia and nitrite present because of the die off mentioned. This should mean that the bacteria populations have been maintained. This is not necessarily so, however. When buying live rock it is always prudent to ask how long the cured rock has been in the ‘for sale’ tank at the dealers. If it has been there for say a week or more the aquarist should not assume the bacteria are fully ready for work.

So back to the question – knowing when the bio filter has initially cycled.

If the aquarist is to use a trickle or canister filter then regular testing of the water for ammonia and nitrite should be done. Sometimes the maturation fluid instructions advise to test for nitrite after a given period – this is because nitrite is the follow-on from ammonia, so its appearance shows that the bacteria that deal with ammonia are present. The nitrite will itself disappear (leading to nitrate) meaning that the bacteria that deal with this are present. The bio- filter is now initially mature. A water change and the slow introduction of fish can proceed, always using caution and checking routinely for the presence of toxics.

Good quality live rock that is newly matured (ie. has been matured within a week) can be assumed to be ready to handle a bio-load. Fish stocking can commence slowly, checking routinely for the presence of toxics.

The same can be said for live rock that has been matured for more than a week – stocking can be commenced slowly. In this case though, more caution is needed as the bacteria populations may need time to adjust to the bio-load. Remember that the longer the period of time since the rock was cured the more the bacteria populations will have diminished – as with all living things they need food. It is important to check daily for the first week or two for any presence of ammonia and/or nitrite. Checking continues after this but does not need to be so regular.

With any system that is using live rock for filtration, it is important that sufficient good quality live rock is present. Live rock affords great filtration, but one rock in a fish tank is going to fail!

The bio-filter, whatever type it is, could also be called the life support system. Without it, or if it is too immature, livestock will have problems or die. As the bio-load slowly increases the bacteria will need to adjust, and time must be given for this The bio-filter will not be fully mature until all the livestock have been successfully introduced and are demonstrating health and vitality.


How To Tell When Your Aquarium Has Cycled
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1 Comment
  1. Well timed. The head of our HR dept. won a tank at our Christmas party raffle and it’s cycling now so IM’d her the link. Thanks for the info! 😉

    jeffry r. johnston’s last blog post..Update: Pom Pom Crab finds missing Pom Pom

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