Total Confusion Regarding Water Hardness!!!

Hello Folks,

I have been trying hard to understand water hardness and it's effects on my fish but I've read so much info that my brain hurts and I still don't think I am any closer to knowing what requirements my tank has!!!!

I have the following fish:
1 CDP
7 Black Phantom tetras
2 Platies
2 Corys

My research has shown that a GH of between 6dH and 12dH should be fine for them. (please correct me if I am wrong!)

I have live plants. Temp is a constant 24/25^oC. (due to warm weather) but heater is set at 22^oC

I did a 30% water change yesterday, using only tap water to refill. Previous to that I had used 10% rain water.

My tank water readings as of this morning are:
Ammonia - 0
Nitrite - 0
Nitrate - 20
GH - between 14 and 21dH
KH - 6dH
PH - 6.8
C12 - 0

My tap water has the following readings (taken from water company website):
Ammonia - <0.16
Nitrite - <0.34
Nitrate - 21
GH - 17.21ppm (307dH??)
KH - 6dH
PH - 7.6
C12 - 0.17


Basically, I want to know if I can rely on just my tap water to fill my tanks (with dechlorinator added of course) and whether it is actually necessary to bother with harvesting rainwater or buying RO water.

Since moving the Phantoms and my CPD in my 54 litre tank 4 days ago, my CPD is now hanging at the surface. Is this related to the water hardness (GH/KH/PH) in any way?? The water in my smaller tank was the same as the big tank as I kept it the same so the transition between tanks was not to stressful.

I am confused!!!! 

FC