I would start with taking the mantis out and doing water changes. Run the lights 8-10 hours to get some algae growing for the urchins to eat. Put timers on the lights so you don't have to turn them on and off.
Skimmer you want a dry skimmate so leave it up where your not getting much foam, over time it will collect in the cup it does not happen right away. And if your tank has not been fed, no fish no light then there probably is not much to skim out.
No food, no fish so having no PO4
is great, nothing wrong with the kit, you said you are using ro
for top off so that is great. Just check the top off water for nitrates and PO4
to see if the filters went bad.
If your not going to take the hobby sersiously then I suggest you save yourself some money and take the tank down before you get fish. I hope your willing to put in 20 min a day for feeding, top offs and glass cleaning and a few hours a month for water changes.