Alkalinity is usually the same as KH.
What is your tap/source water baseline parameters? This means testing it right out of the tap but also after letting it sit for 24-48 hours. See
http://goldlenny.blogspot.com/2007/05/find-your-tap-source-water-baseline.html
Your pH is kind of low at 6.5 but it could be like that from your source or you could have high CO2 levels in your tank since you have good KH and GH levels.
Increase your O2 levels just in case by increasing your surface agitation (raise or change your filter return so it creates more splash). Goldfish need high O2 levels so you can't have too much.
Remember that as your fish grow, the maintenance on the tank needs to increase proportionately. A goldfish increases its size by eight times for every time it doubles its length. For example, a 2" goldfish is equal to eight 1" goldfish. A 4" goldfish is equal to eight 2" or 64 1" goldfish. A 6" goldfish is equal to well over 100 1" goldfish. An 8" goldfish is equal to over 500 1" goldfish. This is also why even a single goldfish needs a BIG tank.
So, if you were doing once a week PWC's (partial water changes) and filter maintenance when they were small, to keep the nitrates low and the KH/GH at acceptable levels, as the fish increase geometrically in size, if you don't increase the tank size, you will have to increase maintenance all the way up to daily PWC's and filter maintenance to keep the water quality to acceptable levels. Otherwise your fish will become stressed, have immune system issues, get sick and eventually die.