how should i add salt?

One tablespoon per 5 gallons - add with water changes, or your salinity will increase. Some folks use regular table salt, but there is debate whether the other chemicals in table salt hurt the fish (ie. iodine, moisture retardents, etc.) Get Aquarium salt at the pet store if you want to play it safe.

Read the label and see if there are any additives. I use Morton's Kosher Salt for my tank. It works well enough. And hey, if my fish want to celebrate Passover soon...

YoFishboy said:

...Some folks use regular table salt, but there is debate whether the other chemicals in table salt hurt the fish (ie. iodine, moisture retardents, etc.) Get Aquarium salt at the pet store if you want to play it safe.

Click to expand...

No, there are some myths about this. Table salt is quite effective at adding NaCl to the water in sufficient quantity for medical treatments. However, 'just adding salt' without any medical reason is poor practice, and has the mythical status of being beneficial. Fish like guppies can tolerate salt, but do not need it, particularly those species that have been captive bred for generations. For fish that truly need brackish conditions, neither table salt nor 'aquarium salt' is adequate since both lack the trace elements and buffers needed to replicate brackish conditions--marine mix is needed for this case.

I'd also like to add to OG's comments that Marine and Aquarium salt can affect pH buffer, whereas plain old NaCl won't.