Your tank is probably going through what most tanks go through. A diatom bloom happens to most systems when they are young. Here is an article
Nutrient and silicate removal will help. Also using RO
water is really important. Do you have a skimmer?
Also Scotter dragonetes (they are not really blennies) are a difficult fish for a young tank. Their natural food is pods
which most young tanks lack. Unless you can get them to eat prepared food (if they won't they may slowly starve), you may want to consider returning them. IMO
Also, what are your water parameter readings?