A friend of mine has just bought a new house that came with an old sink in the garden with a lone fish in it which the previous occupants have left!
Its orange, perhaps 10-15cm long so I suspect a gold fish. Can't tell for definite because when she showed me it yesterday the water was so green and murky the fish was barely visible.
The new owners don't want to keep an outdoor fish so I offered to see if my parents would take him in their indoor tank (with filter). They have two similar sized fish in their already and just lost a 3rd due to old age.
They were more than happy to do so.
Having slept on it I've realised this won't be as easy as just emptying a new fish into the aquarium. Advice on what to do gratefully received.
Don't want to stress any of the fish involved.
An excellent decision not to just empty the fish into your parent's aquarium. It could have been a complete success, or an epic disaster with all the fish being lost!
With the change in environment, the new fish will need to adapt to new surroundings, new water parameters, new food and possibly new diseases.
So starting with the basics; size. You mention all fish will be approximately the same size. This is a good start.
I am assuming they are all a similar variety i.e. single tails. Mixing fancy varieties with single tail varieties leads to the single tailed fish getting fatter and fatter and the twin tailed varieties getting thinner.
Fish that have been in a closed body of water, (sink or aquarium), lose their immunity to some diseases. When a new fish is introduced into a new environment without being quarantined, it can introduce, or it can catch a disease it hasn't been exposed to before.
The new fish needs to be quarantined for a couple of weeks and treated for parasites.
The existing fish need to be treated for parasites at the same time. Use API General Cure which doesn't require the filter to be stopped, but any charcoal should be removed first.
When putting the new fish into quarantine using fresh new de-chlorinated water, do it slowly over an hour by slowly introducing new water into its container.
The other option is to use a water test kit to check that the new water pH is the same or higher than the water the fish is in.
Have a read of this page on how to introduce new fish into their new environment which is covered in some detail.