1) My local tap water is very soft with calcium and magnesium combined below 4ppm. What is a good goal in terms of Ca and Mg concentrations in ppm? I figure I can use Ca and Mg chlorides, but not MgSO4 like Epsom salts, to increase this if need be? I read that sulfates can mess up the determination of CO2 from the knowledge of KH/pH.
2) My local tapwater is baking soda dissolved. The max detected level over the last 3 years was >500ppm bicarbonate. Sodium was at 244ppm max detected.
water is hard or soft depending upon the item of interest you are paying attention to.
Low Calcium and Magnesium means low general hardness (GH). High carbonate means high carbonate hardness (KH).
Your GH might be a little low. I add a powder called "GH Booster" to my RO water to build GH and add baking soda to build KH. You can get some GH booster at http://www.aquariumfertilizer.com/st...cplantfood.php
3) pH is listed as 7.7, but I hear that's on the low side of things and 8ish is more like it. My friend's cichlids love it here with some Epsom salts thrown in.
pH is of course the how much extra H or OH is in the water, where water is H20, or more clearly in this case HOH. It's lagging indicator based upon what's already dissolved in the water. I don't worry about this number as much as the two types of hardness.
What do I need to do to make my water good for plants? Will purification be required, or can I deal with this liquid baking soda and make it work?
Plants such as vallisneria would love your water since they can utilize dissolved carbonate as a carbon source. Most plants can only utilize dissolved CO2 in the water. Some cryptocorynes don't like high KH at all. Many plants don't care very much. For fussier plants, using an RO system is the usual approach to get exactly the water to target at a specific fussy plant.
How often are water changes required in moderate to heavily planted tanks? Maintenance is one reason I am staying away from reef tanks (cost being the main). That and I find myself thinking planted tanks more natural and relaxing to look at every time I see a good one in a store somewhere. Basically I am trying to gauge if I am going to need to be doing large RO/DI based water changes frequently, only do RO/DI changes infrequently and let the plants soak up the nitrogen cycle wastes, or do tap for whatever regimen I choose. Thanks!
If you do Estimative Index, you really want to do 50 percent water changes. Whenever I get too busy to do this for my EI tank, it gets really ugly in a hurry.
I have some natural tanks with low fish loads and medium light that I rarely change the water. These do OK. They are not spectacular, but don't crash on me either when am too busy to do maintenance for a week.
There are other ways to raise plants as well. For example I use EI/4 dosing in my medium light tanks and get by with monthly or bi-monthly water changes.
There is a good overview on the different fertilizer methods at http://www.aquaticplantnews.com/
You may find this useful.
Good luck with this.