This is quite common and not unexpected given your very hard water. The water supplier is adding CO2 to the supply to temporarily lower the pH of the water as it supposedly improves the taste if the pH is nearer to 7. After around 24 hours the CO2 gasses off and the water reverts to its true pH. This is similar to what happens with my tap water which rises from 7 to 8.2 after 24 hours standing.
The end result is that your tank will eventually stabilise at around 8.4 due to regular water changes and you should be looking to stock with fish suitable for very hard alkaline water. It would be advisable in the future to run the water you need for a water change 24 hours in advance to allow the pH to stabilise before it goes into the tank, as using it straight from the tap will cause a yo-yo effect in the water which is bad for fish.
Mollies are the immediate species that springs to mind, or a complete change to small Tanganyikan cichlids such as one of the shell-dwelling species. If you can get a gH/kH test kit it would be helpful to see the results of those tests due to the possible impact on the Gold Barb you have. The pH is a tad high for him, but he may be OK depending on gH/kH readings. If so, you could add a few more to give him a shoal and then add some Mollies. If you do keep the Gold Barb, then Mollies are the only other fish off the top of my head that will do well in your water with them. Tanganyikans can't be mixed with other fish that aren't from the lake except in some very specific circumstances.
If you want to keep mollies you'll need to use the heater, and keep the tank at around 26 degrees. The gold barbs don't necessarily need to be kept that warm, and as long as your house is not too cold will be happy with a tank at "room temperature" with some seasonal fluctuations. Mollies won't cope at all well with that.
So if you let your water mature for 24 hours (so you don't get pH fluctuations when doing water changes) you'll have to heat the water before adding it to the tank, so you don't shock the fish with a sudden temperature change. If you just kept the barbs then that isn't necessary as the water in the tank is going to be about the same temperature as the water in the bucket.
(Personally unless you can find "wild type" mollies- which sadly you probably can't- I wouldn't keep them at all, but then I'm not a fan of most artifical varieties of fish- especially the "balloon" ones.)
In a 100L tank a shoal of 7-10 gold barbs would put you about fully stocked based on the 1cm/2L "rule", which is more of a guideline really. Understocking a tank is never a bad idea when you're just starting out. When the tank is nice and mature and you have a good growth of algae you could get a Bristlnose Plec would be a nice addition, an give some interest at the bottom of the tank. (I'd have a heater set to about 20-21 degrees C if you got one, to keep the edge off just in case you had a cold snap. You'll probably find it only switches on on the very coldest nights).
If you do keep a couple of mollies get either only males or virgin females, or you'll very quickly have hordes of babies. (Females can store sperm for up to 3 months, and drop dozens of babies in one go). They're not shoaling species like the barbs are, so you don't *need* 6+ to keep them happy.