6.5 is a pretty low effluent ph. Depending on your tank size and effluent flow rate, you could knock your tank ph down pretty quick. My reactor works best at about 6.7-6.8 for my size tank and calcium demand. Here's what I would do:

Get your Ca and dkh levels pretty much where you want them. Use Turbo calcium and buffer (baking soda works fine)

Test your effluent dkh; you can use a salifert alk test on the low resolution mode (2ml sample, 1 drop reagent) Ideally, you want an effluent dkh of at least 20; 25 or higher is great. Measure effluent ph as well. Since you have a really low effluent ph now, I'd start raising it by turning down the CO2 flow a bit, until you get an effleunt ph of 6.8. Then test effluent dkh again, make sure it doesn't drop too much, and you have found the right ph for your reactor. What will happen is you will find the highest ph that you can get a high alk effluent at the same flow rate. (speed of drip into your sump)

Once you've got that, you can tune the flow rate to your calcium demand; just run it as is, and if your levels go down, increase BOTH the flow rate and the CO2, keeping a steady effluent ph. If they go up, slow th the effluent and CO2 a bit. Remember, though, that depending on your media, the balance of Ca and Alk may shift. I use ARM, which results in really high dkh (10 or so) with Ca level of 400. (Not real high, but plenty IMO) The point is, your Ca level may drop a bit, but that doesn't necessarily mean you're not replenishing.

This whole process should take a week or two at least to get it stable and really tuned. Once it's there, it's amazing how stable it is. During this time, watch your tank ph, too. I'd check it first thing in the morning occasionally to make sure it's not dropping too low. I've dropped mine to 7.8 a few times as I was dialing I my reactor with no problems, so don't go running for the kalk immediately.

Good luck!