Freshwater to brackish with gsp in tank

[Bumblethepuffer]

Hi Guy's,

Just registered and glad to be part of the group
I've kept freshwater and marine setups in the past, and currently have a freshwater with a green spotted puffer (Bumble) as a single specimen tank.
I need to be converting it to brackish now, and although starting a brackish tank without fish is probably simpler, I have the little dude in there. So my question is, how much marine salt should I be putting in each water change in order to get the salinity to where it needs to be for him? But without over doing it and causing him harm, including the bacteria that also may be affected if too much salt is added in one go.

It's an 80 litre tank And he is a little over 2.5 inches. And yes I know I should be upgrading the tank size soon.

Thanks for taking the time to read my post and welcome all input.

[Pufferpunk]

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