I'll make the assumption you started with dry rock and sand. Either way here is my theory. To get rid of these things, other things have to out compete them. They are like weeds, needing very little to survive. Low PO4 and NO3 is good for them because that is what their competition needs to flourish. I don't see much value in MB7 as a help since it's just a handful of nitrifiers. Look at a biome test test of a healthy tank and there are dozens of different bacterias. I believe this is the key. Like a thick flourishing lawn keeps the weeds suppressed, a thriving bacteria and microfauna population keeps the dinos suppressed. If you have dinos, your tank isn't healthy and diverse down at the micro level.
I had a dry rock tank with zero PO4 and NO3 and the dinos were unbeatable. No matter how much I would beat them back, in 48 hours they were as prolific as ever. Raising PO4 and NO3 had no affect, since there was no diversity in the tank to use it. Tank was taken down to be upgraded.
This time I started with dry rock, but ordered 15 pounds of live sand out of the gulf. While I was waiting for the new tank, I set up my rocks in a trough with heater and powerhead, and poured the live sand all over them. I feed it with fish food every few days for a couple of months. It's been 8 months since I set up the tank and so far it's been just some minor algae problems. Live rock would be better, but I wanted to avoid the pest, and figured the sand would give me most of the bacteria diversity. Live rock or sand is the best way to tip the balance in my opinion, then keep detectable PO3 and NO3 so your good bacteria can hold down the fort and run the dinos out of town. Good luck.
