When you are renting gear, this is always a trial and error process. If you own your own wetsuit(s), then you can measure buoyancy in a swimming pool, and simply add 6 lbs for seawater.
You should take your fins, mask, snorkel, and wearing a swimsuit, go to a swimming pool and figure out what neutral buoyancy requires, first. This is defined as floating vertically at eye level with the water-line while breathing on your snorkel, which will be sticking out of the water over your head slightly. (A lot of people have gotten rid of their snorkels because they don't think they look cool with a snorkel. I hope this is not also your problem, however; I hope you still have your snorkel.)
For me, neutral buoyancy in a swimsuit requires about 2 lbs in a pool. I am "a floater." You may be "a sinker," however. Everyone is either a floater or a sinker.
Then add the dive skins. For me, in dive skins, neutral buoyancy requires about 4 lbs in a pool. You may only require 2 lbs, however.
Now add 6 lbs for seawater. 10 lbs is what you got including your tank at the Keys, strictly by trial and error. This is a fairly general rule, for skinny divers, or young and skinny divers. For me, it takes 14 lbs, for the Keys, with 1/2 mm dives skins and an aluminum tank.
You need to adjust for the buoyancy of the tank. Most rental tanks are aluminum, therefore you will need to add about 4 lbs to compensate for an almost-empty (500 psi remaining) aluminum 80 tank. That is why I need about 14 lbs to dive in the Keys with dive skins and an aluminum tank, and why you need 10, based on the components. Everyone is different.
You can do the same trials in a pool with a 3mm suit. And then add on, for seawater, and for an aluminum tank.
Or you can do the test first with your dive skins on, and then with your 3mm suit on, in the pool, then subtract the difference, and add this weight difference to 10 lbs, which you used in the Keys.
The ideal would be to do the pool trials complete with all the gear you will be wearing, and then add 6 more lbs for the sea. However if you are renting gear, you cannot. But you can, as much as possible, with your suit(s) alone.
And always keep a logbook and write your weighting down, and your suit and tank configuration, so that you have a record. They you can look back into your logbook in the future.
Have fun!