great in salt, complete newbie in fresh

You can use RO water if your tap water is really out of wack, but you would have to use something like kent's ro right to add back some minerals. I would be shocked if 5% of the people on this board use RO water though.

As for planted tanks you need sufficient amounts of lighting for the kinds of plants you want to grow (1wpg on the low end to 3-4wpg on the high) and it also needs to be of the correct spectrum (between 5000k and 10,000k). If you exceed 2wpg (watts per gallon) you should consider injecting Co2 either through a DIY set up or going the compressed gas with a regulator route.

The other area to look at is substrate. I learned about planted tanks from a forum that is pretty much dedicated to nothing but. There nearly everyone uses either fluorite or eco-complete. Here it seems that very few use anything more than gravel or sand for a base. I recommend the fluorite.

At some point you will have to come up with a fertlizer regimen to keep your NPK in balance as well as your micro's. The test kits you will need are ammonia, nitrite, nitrate, Ph, GH and KH. Having been in saltwater I'm sure that you already have some of these kits.

Thats the basics. I'm sure I've missed some things and others will add to and/or disagree with what I've written. My best advice is to read everything you can on planted tanks, both here and on other forums. Aqua Botanic and The Planted Tank Forum are two other sites I use a lot.

Welcome salty.

As a reefer, this should seem fairly straightforward once you get all the info squared away.

Read up on fishless cycling, it's definitely the way to go. The dominant nitrifiers are a different species than those found in marine tanks. There aren't any denitrifiers, treat NO3 as an indicator of water quality. Keep your stocking levels light and aim to do about a 50% water change per week or two 30% changes which is equilivant. This can be fine tuned by measuring your NO3 titer, it should be below 20ppm before your water change.

Now if you go planted, you lose the NO3 marker because the plants will use it.

If you follow this setup, you won't need to fishless cycle, or cycle in the traditional sense at all:
For your setup, start with a very light dusting of ground peat, it's just to provide the plants with a little bit of organic material until the tank gets underway. Onto the peat, add some mulm from an established tank, if you have access to it. Cover this with a good 2" of Fluorite (more if you've got a bigger tank, you can try sloping it 2" front - 4" back), it's not cheap but you can save some money by mixing it in with some Profile or Turface. Although I say not cheap, but as a saltwater enthusiast this'll all seem like peanuts to you!

Check out www.ahsupply.com for PC retrofits. You want a minimum of 2wpg over your tank and this is the easiest and probably most economical way to go. Also, while normal fluo bulbs need to be changed every 6 months to keep a good level of intensity, PC bulbs are good for about 2 years, so divide the price of bulbs by 4 to compare to NO lighting.

Look into pressurized CO2, the planted tank forum here can give you lots of advice on which brands to use and avoid. If the startup cost is too steep, go with DIY, again, lots of info in the planted tank section. For this method I'm describing, CO2 is a must. It's not necessary in the general case, but if you want to go with a "low tech" (less than 2wpg lighting and no CO2) you'll need to fishless cycle first, plant after. Read up on CO2 diffusers/reactors, you can spend all the money you want getting the best pressurized system available, but if you can't dissolve the CO2 into the water, it's a waste.

Okay, not we've got a good substrate, good lighting, CO2, ferts (read the fert. sticky in the planted tank forum, it'll tell you what you need), now it's time to plant.

Draw a floor plan of your tank and get any aquascaping done: driftwood, rocks, decos, terraces, etc. Figure out what plants are going where and get said plants. If you're in the States, there are lots of online places that can ship you just about anything really cheap it your lfs doesn't carry a good selection. Word to the wise, they can ship a lot of plants for about the same price as a few, so don't be shy.

Once you get the plants in, crank the CO2. Run the levels around 40ppm for the first couple weeks. Add no ferts to the water column. Let the plants settle in, remember that they've got the peat and the mulm you've added to nourish them, plus they've got their stored nutrients. After a couple weeks the plants will be hungry, time to start fertilizing. Add your KNO3 and PO4 to bring your levels to about 10ppm and 1.0ppm, respectively. Test these levels every other day and see what you need to add to keep them constant or close to these levels. On the off days, dose traces.

After a week of this lower CO2 to about 25ppm and add a few algae eaters (may I suggest Otos?). Let them have the run of the tank for a week and add a few more fish. Continue doing this a few fish at a time until you're fully stocked. You'll never detect any ammonia, the plants will consume it almost instantly. Your nitrifiers will grow slowly on any excess that escapes the plants.

Keep your final stocking levels light and you'll have great success.

Sterile doesn't work in freshwater. All those nutrients and minerals are needed by the fish and plants and bacteria. If you use RO then you must add the minerals back in. So unless your tapwater is horrible then it will be fine. The less ytou make adjustments the easier it is to deal with a crisis situation and the easier it is to make water changes.

Yeah, what he said. RO's a little too pure for most applications, unless you have real problems with your tap, it's not worth it.

I used to use RO in my reef tanks- mixing the brew in 2- 35 gal new trash cans (oh, what fun).
Now I'm trying my hand at starting a planted community tank but my tap PH is a whopping 8.2! Being I want a planted tank full of tetras- my goal is 6.8-7.0. I would need big bottles of chemicals to correct the PH. Somehow- when you read the bottle & it says poison- and you are having to dump lots of it in to achieve neutral- what's the best solution? I have store-bought Ro & it's at 7.4- much better than my tap. I was planning to use that- until I read here.
Suggestions?

You can mix RO with your tap water, that is one solution. You would need to find a desired mixing level to get a pH and KH and GH you wanted. But instead I woudl think about trying to add some CO2 into the system since you say you want a planted tank. Depending on what levels your water is (KH etc...) you can easily lower the pH of your water since CO2 will create a more acidic environment. Plus CO2 helps plants grow faster.

I would ask this question over in the planted tanks forum as they will be much more helpful. But you should know the pH, gh and kh of your source water first as they will want to know those.