In order to get high pressure fittings snug, we use a thin wrench to hold the base fitting in place and we make small tightening adjustments with another wrench on the nut.
If a tube fitting leaks, it can sometimes be resealed by just tightening. You must use two wrenches, a 13/16-inch wrench to hold the base, and a 7/8-inch wrench to turn the compression nut. The 13/16-inch wrench will need to be thin so as not to interfere with the compression nut. If this doesn’t work, disassemble the fitting, grease liberally with silicone grease (the ferrule and the threads) and re-tighten firmly.
The base O-rings should be gently compressed to achieve a good seal, and may be damaged by over-tightening.
Many times, the ferrules will weep and simply need to be re-tightened. You want to make sure that you hold the base nut in place and use the scissors tightening technique outlined in the manual. Tighten the nut hard while holding the base nut firmly in place.
Above are the two most typical leak paths, tightening would address a leak with the light blue path.
You can also lightly sand and grease the ferrule and fitting. This might help make a better connection.
The dark blue path can sometimes be fixed by tightly wrapping 2 wraps of Teflon tape around the bottom of the tube. You want half of the tape on the tube and half of the tape on the base of the ferrule.
If neither of these methods work, you may have to either live with the leak or replace the tube and fit on new ferrules and nuts for both sides. Below are part numbers for replacements. Contact your distributor to obtain replacement parts https://www.spectrawatermakers.com/us/us/storefinder