First I am not sure what HCl pH adjusters are or even how to go about looking them up, a link would help. Tap water can be a problem, EC is not a good measurement of water quality. EC (electrical conductivity) cant tell you what is in the water, it just tells you the total electrical conductivity of everything that is in it combined. It cant break it down for you.
(I have a dollar in my pocket, does that mean I have a bill, 4 quarters, 100 pennies and/or any combination, all you know is that I have a dollar total.)
RO is good, but expensive just for a few plants. EC still cant tell you much there either, but at least you can be sure that most all the contaminants and excess minerals are filters out.
nutrient temp should be good if it never gets higher than that. 25 gallons seems a bit small for 50 plants. Are you needing to add much replacement water to bring it back up to the 25 gallon level? Is that water pH adjusted? Lastly a .2 or .3 change is not something I would worry about. Lettuce should do fine between 5.5 to 6.5. If it slowly rises you can set it low at 5.5 and let it rise to 6.5 before re-adjusting.
I would go with a larger reservoir, and better water quality (If you can). Also I didn't ask but changing the nutrient solution helps too. The larger the reservoir, the less you should need to change it (due to nutrient depletion). Nutrient depletion can also be a cause of pH changes, due to the change of elements/ions in the solution.
P.S. With commercial
nutrients they would have pH buffers in it, with home made nutrients that's up to you. What's the pH of the tap water before you add any nutrients at all.