To save them...continued?

You've had your aquarium running with fish for 6 weeks now so your ammonia should be fine, but what are your nitrites and nitrates. Your other fish could have died of nitrite poisoning, since they died at about the right time for the nitrite spike in your nitrogen cycle. If you have nitrites higher than zero now, or nitrates higher than 20 ppm, then I'd bet that was the cause. A pH of 8.0 is also a bit high. Do you have coral or sea shells in your aquarium? If you do, remove them slowly (one every couple of days) because they are buffering your pH up too high. If you don't, you might want to add some driftwood to your aquarium or peat to your filter to bring your pH down a little. You want it to change slowly, but ideally you should have your pH between 6.8 and 7.6. What is the pH of your tap water or the water you use for water changes? If it is too high, you may want to soak some peat in it or add some pH down before you add it to the aquarium. Quick changes in pH are bad, so whatever you are doing, do it slowly. And if your pH was orginally lower and went up to 8.0 quickly, that could also have been the cause of your previous fish deaths.

Try not feeding them for 2 days and give your tank a chance to stabilize. It is really easy to overfeed.

The 2 hours in the bag is waaaaay too long to go without oxygen! The adjustment to the new water from the bag should only take 20-30 minutes.

When I acclimate my fish I float the bag long enough to get the temps the same between my tank and the bag, dump the fish into a fish net over the sink and then place it in my tank. The petstore water may have some type of infection in it - you don't want foreign water in your tank.

I also put a dose of Coppersafe in the tank because it doesn't get filtered out by the carbon filters.

And I turn out the lights on the tank until the next day to allow the traumatized fish to relax!

I agree that you need to do a weekly water change. Especially in such a small tank. The smaller tanks need to be monitored closesly...especially when new. Small problems can get huge fast in a small tank.

Also, try taking a sample of your tank water to you pet store...most are most than willing to run a complete array of tests on your water. Maybe the water is off...but you're not testing for it.

Lastly, if the fish from that particular pet store still keep dying...maybe you should find a new pet store.

Good luck!

Sounds like your tank is overstocked. What type of goldfish are they? A 30 gallon would be okay for one fancy goldfish, two would be a tight squeeze, and 6 is just way too many.

When you say ammona is "fine" - what is the exact number? And how about nitrtites?

Also, 25% every two weeks is NOT enough, even if you have powerful filtration. Regular cleaning should be done, 30% EVERY WEEK. With your amount of fish in such a small tank, you should be doing 30% every few days.

This is why your fish are dead / dying.

Edit:

jmrob29 - it is sometimes recommended that fish be slowly acclimated to their new tank water. Rather than just letting the temperature adjust, then dumping the fish into a net over the tank and letting it go into the tank, water from the tank is slowly added to the bag so the fish can used to the new pH, KH, GH, etc, not just the new temp. 2 hours is not a long time to be a fish bag that is properly filled with air and water, and it is not too long for proper acclimation (when the bag is open, oxygen enters the water). Then the fish is netted from the bag and placed into the tank, and the water from the bag is discarded.

Also, you should remove your activated carbon before dosing with coppersafe. It is safe for biological filtration, in that it won't kill your nitrifying bacteria, but leaving your activated carbon filter in place renders the coppersafe ineffective.