Man they took a hard hit, sounds like. Algae wont kill roots, and my experience with root rot, {pythieum} has thankfull been limited to two times. Once i caused it on purpose as an experiment by reusing the same rockwool for four crops.

Next time was in dwc buckets with too warm water {75} I managed to save both crops with lots of tlc but the pythieum is a systematic disease. Which means that a plant that is badly infected will always have the disease. It cant be "cured", and i've found that even tho plants can recover enough to stay alive and flower, and go on to become mamas and clones, they are always smaller and weaker plants when compared to non infected plants. The potency is lower, taste is worse, and they generally are an inferior plant compared side by side to a healthy plant. Just my 2.