I agree with
@Sallstrom.
One other thing that sorta jumps out at me is the amazing coraline algae on the old tank vs the naked substrates on the new tank.
If I could go back in time I'd suggest scraping down the walls of the old tank and "dosing" that material into the new tank.
Imagine with me for a little bit....
Assuming all your levels are staying the same as they were in the old tank (not a good assumption, which is why I agree with Sallstrom on needing nitrate data) and "all else remains the same" then what has obviously changed is the amount of surface area that your CUC is responsible for and the nature of that surface area.
Before, let's say it was 60 cm2 per snail before. Now it has to be more like 600 or 6000 cm2 per snail. It looks like all open space to my eye. Those are made up numbers just to talk about proportion.
To make matters worse, I see that your CUC was obviously focused on grazing the massive "coraline fields" of your old tank. Undoubtedly the coralilne algae growing underneath would have a negative impact on whatever algae the snails were grazing. This impact would probably make this epiphytic algae grow more slowly, weakly and thus more suitable for grazing.
The fact is that your CUC is outmatched in the new tank – perhaps by several orders of magnitude.
Your algae now is growing freely on essentially naked calcium carbonate surfaces which bear plentiful amounts of adsorbed phosphates for direct use to nearly any/all algae that can settle on it. They probably have little need for the PO4 in your water, even if they're using it.
I suspect you're going to need a substantial upgrade to your CUC for them to keep up with this algae.
An urchin will be a great addition – and make additions slowly – but I would be prepared for adding more snails. Perhaps a lot more. Keep variety high. If you can add more fish, consider good generalist herbivores like Rabbitfish.
