If you are new to salt tanks I wouldn't go any less than a 20g IMO
. Shoot for the biggest tank you can so it will be more stable and more forgiving.
I started with a 10g and even being diligent with water changes I still had swings and wasn't entirely sure what I was doing. I eventually had enough and bumped it to a 20g where it is now much more stable and healthy.
If you are up for it id highly recommend a sump. It's much more efficient and clean than an HOB
. I'm currently running an HOB
(sump in the works) and it gets dirty very fast.
With salty, a lot of pre planning can save you a lot of hassle.