I recommend a water change at this point depending on the progression of nitrate, alkalinity, and calcium. Often new systems will need a large water change at this point to maintain the balance of buffering ions and lower nitrate buildup.
First things first, you are ready for the next step, which is to continue doing nothing.
You need to let the diatom bloom run its course, which it sounds like is happening already. Great sign!!! In the meanwhile, now is the time to begin your testing, buffering, and supplementation routine. All marine systems are heavily dependent on calcium and alkalinity, so you need to be focused on keeping these levels where needed. Fish only tanks are just as dependent, perhaps even more so, so regardless of the type of system you need to test for these and adjust as necessary.
I personally maintain my alkalinity at 8 - 12 DKH and calcium at 400 460ppm. I use Kent Marine Super Buffer DKH and Kent Marine Liquid Calcium Chloride as supplements to do this. Another good option is the BIonic 2 part additive, which is also very popular on this forum. Both work great, just be sure to use one or another.
You want to begin this now, because you are in the diatom bloom maturing phase. When this diatom recedes, you want coraline algae to begin taking its place. If not, you will likely experience a prolonged period of problematic algaes, such as hair algae and bubble algae. This should be your priority this week, to learn how alkalinity and calcium work and become comfortable adjusting these levels.
Here is a more detailed article which I posted on the topic: http://www.tropicalfishkeeping.com/m...-marine-33079/
The marisys 240 is an interesting filtration design. We really need to discuss it in greater detail also. I believe this is a 38 gallon tank, correct? Or is it a 55? What are the dimensions? You have what you need, but you may be improved to make some small adjustments to how the system is used. Can you provide details on what you plan for this tank in the future, in terms of livestock?