I havent been taking care of fish long so I'll be as helpful as possible...

I have a 20 gallon tank with the following fish:
1 Algae eater
3 platys
1 pink kissing gourami
1 striped gourami (Colisa fasciata ?)

The past week or so the kissing gourami was chasing the striped one around to the point that the striped had to hide repeatedly. This has worsened over the last few weeks. Last night I did a regular water change (approx 30%, added water conditioner and cycle), and this morning I found the striped gourami lying on the bottom on his side breathing heavily. Since I found the striped gourami this morning I have done the following:
1) Found a new home for the kissing gourami (he has a tendancy to bully, so I have just given him up for adoption)
2) Had water tested by LFS (more on this later)
3) LSF recommended erythromycin tablets, because they think the stress of being chased caused him to develop a bacterial sickness. To use these tablets, I had to remove carbon filter/stage of my filter system

I have tested the water myself this evening and these were the readings I got for a 20 gal tank:

Ammonia: 0.2
pH: 7.8
Alkalinity: 300 (high I know)
Hardness: 150 (not sure what unit this is)
Nitrite: 0
Nitrate: 0
Temp: 70f

It seems I have to do something about the alkalinity, but should I wait until the striped gourami gets better?
How do I fix the alkalinity and hardness?
Any other suggestions on helping him (other than a hospital tank). The other fish are doing well, and as I have mentioned, the LFS has adopted the pink kissing gourami that was bullying the others, so that stressor has been taken away.

Thanks for any help