I'm a first grower too had kinda the same problem like you do. You are lucky because it doesnt seem to effect your plant too much.
For me its pretty hard to decide what exactly caused my problem but I believe this factors did (in order).
1) I overwatered them without even realizing it. My new method is just giving them water to run off from the bottom and then waiting till the upper layer of soils is completely dry (3cm or 1 inch) before watering them again. (as I heard overwatering is number 1 rookie mistake) Serious overwatering can cause problems what is similar to some defeciency as the roots cant absorb certain minerals and nutritients.
2) have a water with 8.0 ph, even after nutes it was 7.3 and soil ph wasnt the best either(don't remember the exact value). So check your ph too, you can buy a ph tester for 30$ which is not too much investment as probably one month of electricity cost this much.
3) Ca-Mg def. I would advise you to start gentle, probably half the dose because theres a good chance its an overwatering or ph problem. And yeah, filtered water is no good. Use tapwater and let it sit for at least 24 hours to let certain chemicals evaporate off.
Thats what I learned from the issues with my plants. (feel free to correct me)