Bruce Leyland-Jones
Fish Aficionado
You should NEVER 'need' to add all your fish at once!It helps you beneficial bacteria keep up. Add too many fish at once and it won’t be able to handle it all. If you need to add all your fish at once, you can just make sure to test it and do water changes whenever there is ammonia reading
That said, I have seen posts advocating such a practice from members of the Mod team. This ties in with the myth that once your tank is cycled, then that's it. A 'cycled' tank is always cycling.
'Cycled' simply means that the tank has reached a stage where the beneficial bacteria are now effective in managing the waste ammonia and by-products effectively. This ability can fade away, if the tank becomes over-stocked, for example, or the beneficial bacteria is reduced by a bad water change, etc., etc..
With cycling, you are establishing a delicate and balanced ecosystem, (and the smaller the tank, the more delicate it is).
Adding too many fish WILL upset the balance and fish will die and posts will be made in the Forum Emergency section...again.
This is especially true when plants have been ignored, or dismissed as being too difficult. Even the smallest of tanks can have plants and floating tanks virtually need no care whatsoever. Plants can act as a 'buffer' for so, so many mistakes, thanks to their natural ability to absorb ammonia and ammonia by-products.