As soon as I had the filter running and chlorine removed, moved the 4
fishes
on and had survived very well (they are very tough), I think they must
helped to accelerate in some ammount the cycling of the tank.

You can use Amquel to detoxify the ammonia and still have it available to
cycle your biofilter



The actual fish population is the following:

4 kois? (I have become fond of them
10 neon tetras
2 Angelfish
2 Black Ghosts
2 female Betta Splendes

Soon to be zero neon tetras, your koi should find them very tasty. Koi are
coldwater fish, and the rest are tropicals. I'm not sure how wise it is to
mix them.


I think it would ver very logical to remove the 4 kois, but I need first
to
know the cons of maintaining them on the tank (they seem to do very well,
are very acctive, have very intense colors and are allways hungry).

My plans are to add one ancistrus, a Male Betta Splendes and a pair of
Blue
Gouramis. I know I'm mixing hard water fishes with soft water ones, but I
believe the tap water in my city is neutral to soft and all species will
do
well.

In case I had to forget the bettas / gouramis, what species would you
recommend to complete the tank?

Ok, I think it's enough for the first mail, in following messages I will
continue to describe the details of the tank. Will appreciate any comment
/
advice / etc that could help to achieve success and improve the tank.

My advice - If you have a garden, dig a hole and make a pond for the koi and
keep the tropicals in the tank. You can even use pre-formed plastic shapes
you can get at garden/d-i-y centres. Or get another tank for the tropicals,
and keep the koi in the first tank.

Marcus