I have a 250 gallon system in my house, and I used nearly 150 lbs. of Marco Rock for my DT and 100 lbs. of pre-established live rock from a former system in the sump. It's been running for 6 months, and I have no problem with PO4. The only problem I had was that I hadn't rinsed my rock first, so my the first week or so my tank was milky.
In the past, I've dropped Marco Rock into an established system and had small algal blooms, but as soon as I saw them I just did a 10% water change within 24 hours and the system corrected itself. They may have also been from me disturbing my prior bacterial colonies and DSB life. Just keep in mind that a little algae is nothing to worry about in the grand scheme of things if you're adding a large amount of clean rock. It'll fade out within a week or two as your system builds up the right amount of bacteria and new life springs up to fit the bio-load. You'll also find that a small algal bloom will give you a bump in pod population if you just leave it alone. Just remember to check your flow around the new rock so you don't get a cyano outbreak.
