I recommend adding fish only after the tank has had a bit of time to settle. The system should be given a little time to find a proper balance. So much has been done in such a very short time in the set-up phase that it can take some time before the water gets into a long-term equilibrium.
Give the tank a little time to get itself an initial balance before fish are added. The best time for adding fish to the aquarium is often a controversial point between experts. Some live fish stores have no problem to bag and sell the fish right along with the starter kit. Admittedly, this can be done, assuming you have enough experience to get through the set-up phases rapidly. The water must have the right temperature right from the start. Everything must go perfectly to have success with this method.
I never recommend this for a true beginner. There are too many unknowns that can slow the set-up time in the beginning. I don't feel it is fair to stress the fish while they wait in the bag. The fact that the chlorine remover is basically instantaneous makes this possible, but not very practical.
If adding fish must be immediate, be sure to use a full featured water conditioner. It must not only neutralize chlorine and chloramine, but also chelate heavy metallic ions that may be in the tap water.
It also should have a fish bandage - an artificial "slime coating" - to protect the fish from abrasions and slime coat loss during the netting procedure. Slime is lost every time a fish is captured in a net.
If adding tropical fish into a brand new aquarium installation, they need this protection from the environment as it settles.
It takes about a day to settle an aquarium. This is a fair amount of time to get the temperature stable and know it is has reached its set level. Any adjustments to the heater should be done before the fish are added.
Other substances such as pH and hardness may vary a bit in the beginning, so you want to give the conditions time to come to a balance before introducing the new fish. I tend to wait a day or two, just to make sure everything is going as expected.
Some knowledgeable aquarists prefer to start an aquarium without adding fish. They artificially cycle the system with inorganic ammonia only. They are convinced that they can successfully mature the aquarium without fish. I admit it can be done, but personally I believe this is a waste of time.
It can take up to six weeks to do this, with or without fish. That seems an awfully long time to stare at an empty aquarium while it artificially cycles.
The whole idea for most fish keepers is that the fish are the reason for having the tank. Why use artificial means to initiate the system when nature is much more beautiful? Mother Nature will do it efficiently, naturally and provide a living picture while it does so.
Pick the right fish and let them age your water instead. Give the tank a couple of days before adding fish. They will do everything the artificial start would accomplish.
This way it will be natural, using the fish to provide the ammonia and
letting a low number of hardy fish live through the process. This is
where many problems can arise. If you understand what is happening, are
patient and add the correct types and quantity of fish, it should not
be dangerous to them or the system.
The use of fish to mature the aquarium water and initiate the Nitrogen Cycle to populate the biological filter is not too tricky. The natural methodology adapts the entire system to natural processes right from the start.
You can give yourself a big edge with a biological filter augmenter like Nutrafin Cycle to give a more effective safety cushion. Also, successfully maturing the aquarium is the first accomplishment to make any novice aquarist proud.
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Beginning Aquarium Tips