With tap water like that you are the envy of aquarists everywhere. Much easier to harden water than to soften it.
You have two primary concerns:
GH: since you are doing plants and CO2, boost your GH with ferts such as the ones on
http://www.aquariumfertilizer.com/, or
http://greenleafaquariums.com/aquarium-fertilizer.html, or Seachem Equlibrium, etc. Tom Barr suggests always boosting by 2 dGH regardless of the tap water, but some people with hard water report success without doing so (especially in non-CO2 tanks and regular water changes to replenish). I also believe Roy when he recalls Tom Barr recommending at least 5 dGH. For most situations, it is equivalent advice.
KH: primary concern is PH crashes. With low KH, PH swings more wildly given the same tank event (dead fish, stirring up substrate, skipping too many water changes, too much fish waste from overcrowding, CO2 "experiments", and, ironically, even adding stuff in an attempt to raise the KH).
You have probably come across this info many times before, but if you work like me you ignored it because you had hard water. Time to go brush up! There is plenty of info on the net about PH, GH and KH, and it should be covered in most every book on fish keeping. One such resource:
http://www.thekrib.com/Plants/CO2/khgh.html, from a venerable source.
BTW, plants can use up KH hardness as they grow. KH is also called
carbonate hardness, and plants can extract the carbon. Some plants, such as Vallisneria, are really good at it, and tend to do less well in soft water tanks, all other things equal. But most plants can take a tank from KH=2 to KH=0 over time. Something to be aware of if you tend to be bad at keeping up with water changes.
You can also use things like baking soda to boost KH. I've never done this, and I've seen reports that it is a pain (the tank "bounces back" and you've put your fish through a PH swing for nothing). Crushed coral or other similar substances can boost KH slowly, but I don't think using it is universal practice in planted tanks, even in the soft water locations in western WA.
That said, Cory @ Aquarium Co-Op does throws a bit of crushed coral into his display tanks to boost KH -- a preventative measure against PH swings. If I owned a store, I'd probably do the same, because display tanks are put through more bio-load stress than a typical home aquarium. If you go the crushed coral route, I'd suggest putting it in a bag in your filter and not mixing it with substrate. That way you can adjust the amount conveniently.
I tested a non-CO2 planted tank of mine recently to find KH=0 (my tap tests with a GH and KH of 2). The PH was fine, and so were the fish. It wasn't an issue because the tank was stable and lightly stocked, but I took it as a sign that I should at least keep up on water changes. When I do, KH doesn't drop to zero.
I was going to recommend looking up your city's water quality report to figure out why your water changed, but I see Roy did the dirty work already.
