What knot can I use to connect to an eye bolt without losing tension?
100% Trucker's hitch. It's the only knot I'd ever use in the situation you described:

..so I don't lose any of the tightness after pulling it.

From the linked page, above. Emphasis mine.

The Trucker's Hitch (also called the Lorry Knot) is a self binding knot. It's most common use is for tying loads to secure them to a fixed point. For example to secure a canoe to a car top, a tarp to a trailer, or any application where a very tight rope is needed. This knot has many advantages. It is non-jamming, can be tied anywhere in the standing part of a line, and even under the most severe tension, these knots remain easy to untie.

1) You must first tie a quick release loop above the tie-down point. To do this, create a small bight with the running end, leaving plenty of tag to work with.



2) When this is complete, pass the running end around or through the tie-down point then pass it through the quick release loop.



3) Pull down on the running end to tighten.



4) Secure the knot with two half-hitches.



5) Pull tight.



You can also find several videos on YouTube demonstrating how to tie the knot.

Learning this knot is a game-changer.