Exactly what I was thinking. I always thought tank water has too much microbial life that will out compete and consume the photo.
I was thinking this would happen as well, but figured this was an experiment. If it did die off then I learned my lesson. But I'm not seeing any appreciable die off. Cultures are still flourishing... when they aren't nutrient restricted. Just need to tune the dosing regimen to ensure harvesting enough every week.
I would like to eventually do an automatic phyto dosing system, but the thing I am trying to reconcile is how to deal with the volume of phyto in the dosing tube. If you do a continuous drip system 24/7, its not a big deal as long as the tubing is short enough. Too long and the phyto will start to settle before they get to the end. If you only dose during a specific time, then the phyto stuck in the tubing will die. Maybe need a way to suck the phyto back. Now I'm adding in a dosing pump. Now two pumps, one for dosing phyto, the other to pull top off water.
Second issue is dosing rates to ensure you don't crash the culture. I get around this by switching which culture I dose. You could setup inlet and outlet 3 way, or N-way for N-1 cultures, solenoid valves/manifolds and use a microcontroller to switch between cultures. Becomes very complex with a lot of failure mechanisms. Not impossible by any means, but if I was to make something this complex, I will need to add in a lot of redundancies.
So for my system and dosing regimen, I would need two dosing pumps, one for top off that doesn't need any special coding. The second dosing pump for sending phyto to the tank would need to be coded to pull back the volume of phyto in the tubing every evening. There would be two 3-way solenoid valves, one for top off, one for dosing. I would code the controller to switch between cultures every 7 days. I'm going to stick with my manual method for now. I want to make sure phyto dosing is what I want to do long term before I spend time and money to develop an auto dosing system.