Ok, so at least if it’s not changing color, then it’s not a contamination issue, but if the cells themselves are dying and sinking, then, it’s possible that the cells are clumping, which might indicate a bacterial issue, and, while there’s lower possibility of this, it’s possible that there might be a bacterial load in your starter culture as well.
Eldredge also gave some really good advice as well, your culture should be somewhere around room temp (try for 70-75, no heater in the culture water). Also make sure your culture water and fertilizer are sterilized before use (you can heat to sterilize it, but make sure it absolutely comes to room temp before using it, I personally don’t do this but use bleach and sodium thiosulfate). Also, like Eldredge said, when you first start a new culture you want to give it lower light, then as it gets denser you can increase the light, then, the time you are ready to split the culture, your iso culture will now be used to your lights and water, so you can keep the same light intensity.
For myself, I had to go through about 4-5 different vendors of iso before I found a vendor that provided a batch that stayed viable, and now I’ve had the same iso culture going for 3 years now. Remember that yes, most of the time crashes are our fault, and most vendors will do the best they can to give you healthy strains, but accidents happen even with vendors. On top of this, iso is one of the more fragile species, so it makes for a great time.
Maybe try a different vendor for the iso and see if you get a better result (my recommendations are suncoast marine or Florida aqua farms, but the unfortunate part is Covid hit both of those vendors really hard, and I’m not sure if they fully recovered from it). If those don’t work, and you have the cash, you can get a pure strain from the university of Texas (UTEX), and, while that’s the highest probability of getting a pure strain, it’s costly (can run over $200 for a 50ml starter culture).
In my experience (at least):
If it turns white, then clear: your phyto bleached out and died, which means a chemical in the water stressed and killed it (like excess bleach or alcohol), or your light was too strong and it stressed and killed the cells. This can also happen with water that’s too hot.
If it changes color (mostly to green): the culture was contaminated with another algae. If it turns yellow though and it’s not supposed to be, the cells died and they yellow is bacterial takeover.
If you see brown or black buildup on the walls of your phyto container: you have a higher than normal bacterial load, which is causing the cells to clump and die. With a healthy starter, it can usually survive for a couple of days with the clumping building up until it crashes (so you want to move quickly when you see this to salvage the culture).