I used the standard stenner grey tank with the integrated pump mount. Acid vs. Pump 1:0.
In the spring I'll remotely locate the pumps from the chemical tanks.

lesson learned.

Those tanks are not sealed so they emit fumes all the time. Bummer you had to find out the hard way.

What if you dilute the acid? Doesn't that cut down on the fumes?

Yes, fumes are cut down at lower concentrations of acid, but they will still be emitted, just at a slower rate. Even acid in jugs has vented caps so one really needs to store acid in a well-ventilated area. The problems with storing acid tend to be worse than those with storing jugs of chlorinating liquid though even the latter is an issue if not in a well-ventilated area.

See Figures 1 and 3 in this link which show that for the concentrations and temperature that you are dealing with, the (partial) vapor pressure is roughly proportional to the concentration. So diluting 1:1 to get to 15-16% concentration will cut down the vapor pressure and probably the fuming rate roughly in half.

You could wrap the tank opening area (i.e. lid lip area) with a towel that was soaked in lye and then dried out through evaporation. I think the fumes would get neutralized when they hit the towel. Then again, this might be a harebrained idea.

So would this be a problem for CL in the tank?

Chlorine will also outgas and concentrated chlorine, as in chlorinating liquid, will have vented caps. As the chlorine degrades, it can generate oxygen gas that when released will also end up releasing some hypochlorous acid. This is especially true with inferior chlorinating liquid (see these graphs for examples). However, it doesn't tend to outgas as much as acid and it isn't as damaging as acid, but it can still be a problem. So whenever you have either acid or chlorine in containers, you will want the area to be well-ventilated and won't want metal close nearby. We've had reports of people having these items in their garages where bicycles nearby got damaged from the fumes, though mostly this has been with acid.

Usually bleach in 6% or less concentration is in bottles that may not have vented caps and tends not to degrade as quickly and therefore not to vent as often anyway (even if not solidly capped). So people often have household bleach under a counter and usually don't have serious problems with it, but it's still a little risky.