Starting to see some of the burning I've seen in previous grows and, due to this, I've been paying special attention to the pH in the trays themselves. I've noticed pH in the trays has been about 6.5 and, when I've found this, I've been pulling nutes out of the trays and replacing with fresh nutrients (trying to keep the nutrient level the same when replacing as to not drown the plants).
Upward growth has slowed to a halt at this point and the flowers on this /very/ compact plant are starting to mature. The flower size is looking fairly unimpressive but should prove as a good place to improve from. The Sour Diesel next to it is another story! ha.
Your best bet is to get beneficial organisms for roots. Someone mentioned hydroguard. I use this and have no algae issues. Having good organisms mean that bad organisms will be out-competed and won't be able to get a foothold.
Clean out your res very well again and start root bennies BEFORE algae sets in!