how much salt for 1 ft tank ?

Can any one help me to answer the few question below ?

Thinking of put in some salt, but no confidence that how much to put. Currently, only put very little salt, i.e. 1/4 spoon for the 1ft tank.

Is it a must to add salt to the guppy tank ?

How much salt to add to a 1 ft tanks ? ( about 20 liters tank, 30 x 30 x20 cm)
1 tea spoon for 20 liters ? normally, how big is the tea spoon size ? 5ml ? 10 ml ?

Is it ok or necessary to put a small placo in the tank together with the guppy ?

Is the placo consider scaleless fish, can it tarhan the salt ? I read from somewhere that scaleless fish cannot put salt ?

Thanks.

I add a bit (2 to 3 small pieces) into the tank.
Salt is not necessary for any tanks. Hence i dose just a bit.
But salt can help to reduce nitrite level if its too high.
Pleco is fine with guppies. I keep bristlenose plecos with my guppies.

[fadedloves92]

pleco can tahan salt.
but pleco grow very fast.
my normal pleco brought home abt 1inch 2months later 3-4inch already

it's not necessary to put salt into the tank, salt is only used when your guppy is unwell. yeah plecos can take the salt.

[AngryMan]

I have never put salt in the tank for my guppy. I think guppy hobbist also seldom do that. Maybe when they are sick, occassionally some might put
salt.

Thanks for all bro for the advise.