Thanks for the quick responses guys. I'm going to get some cal-mag in the morning. I water once a week. Usually half a gallon. Is that to much?
I don't think you need calmag. What kind of water do you use? Tap water or RO? If it's the latter I could see adding 2ml/gal calmag to replace the lost minerals from filtering. But, I wouldn't add calmag just because it's common. If you don't have a ca nor mg def, there's no reason to increase them.
When (not if) you have ca or mg def, you can treat them specifically with gypsum and epsom salt, respectively. The only time you'd want to use calmag to treat a deficiency is when you have both ca and mg def. If you have gypsum and epsom salt on hand to treat individual deficienies, you can use both to treat ca/mg if they occur together.
Most calmag products contain N (due to the calcium supplied as calcium nitrate). Your plants might benefit from more N. But, you can supply that more naturally (supporting the soil biology which contributes to the plant's uptake of nutrients) with Alaska Fish Emulsion (home depot, lowes).
Calmag's not necessary in soil if you amend dolomite in the soil and your water has some ppms (not RO water).
Regarding quantity to feed. Once a week sounds infrequent to me. I'd improve drainage to your next grow. Try to get down to 3 days. Feeding with 20% runoff is always a good idea. It's either that or flush (heavy) when switching to flower. I think the infrequent "mini flush" each feeding is less stressful to the plant.
Whether you feed a half gallon or more is up to you. You just want it to be enough that it produces runoff that's about 20% of the container's volume. (A 3gal container? Get 3 qts of runoff. Mix enough nutrients to do that.).
