Hi Lesly! Welcome aboard the crazy planted tank train..
Here's the thing with lights.. if you want to have macroalgae, they are pretty forgiving in terms of light. PC and VHO should be plenty for them. Actinics probably wont help them out much with growth, I would go for more in the daylight range - 5000 - 10,000K unless you want to grow some deep water loving species. Some people seem to be finding that a few species of macroalgae have different growth patterns and color variations based on the amount of light. Not totally unexpected, but pretty cool.
In my experience, you wont need any CO2 injection to keep the macro and seagrass happy. They seem to be able to pull their carbon requirements for growth out of the other molecules in seawater (most likely alkalinity, so you might want to keep tabs on that in the tank if you end up having a high bioload of macroalgae).
Macroalgae and the vascular saltwater plants (like the seagrasses) do uptake some oxygen at night and it will play around with your pH levels through the photoperiod. The easiest way to combat both of these that I have seen suggested and tried, is to simply continue running a cheap protein skimmer or another source of aeration. This will help to avoid major swings in either parameter during day or night. You will have to test your tank when its up in order to be sure that a skimmer/aeration is enough to help.
Most people find that, long term, real high-powered (and high cost) skimmers arent really useful on planted tanks and that they produce very little skimmate. I know that I only rarely have skimmate, and if I do see some being produced its usually an indication that something is off in the tank.
There are a few molecules coming from the Caulerpa species that could be considered toxins in high amounts but I dont think they are particularly harmful to the fish. One way to get around the Caulerpa headache is to keep it pruned back and to monitor your nutrient levels in the tank to be sure they dont bottom out.
Some test kits you will likely want to consider for the future: alk, calcium (for the calcified macro species), iron, pH, phosphate, nitrate.
I think a 150gallon project is a really tempting idea! Lots of room for different ideas and different approaches to aquascaping. If you find, after searching around the board more, that you are interested in seagrass as well, you may need to bump up the light levels a bit. But I'm really not sure! I'm not good with VHO and the tanks I have grown seagrass in are all less than 16".. I think a 150g is much taller (?).
Anywho.. feel free to ask lots of questions and search the board. There are a number of people here with plant heavy tanks that are just getting started like you. Its a bit of a quiet group on this particular forum.. but we will pipe up if we're sure we know something!
Have fun!
>Sarah
