You should really have a filter that's x10 your amount of water for Goldies. The one that came with the kit - as you're finding out - is struggling a bit. I'd recommend picking up a canister filter; it will serve you well in the long run.
Regardless of how much maintainence you do or do not want to do; I strongly suggest you learn about all the processes involved in the hobby now, and as much about the particular fish you want as well.
Coming into a hobby like this with the mindset of wanting to do as little work as...
I'd imagine mixing Tropical and Coldwater fish mostly results in a loss for one, or both parties.
Optimum water for the trops will result in an increased metabolic rate for the coldwater fish, thus decreasing their lifespan. Optimum temperature for the Goldies will result in decreased...
Medicines containing Formaldehyde (of which can be found in NT Labs no.9) can bind to, and reduce the Oxygen levels in the water - generally it's advised to run an airstone during medication to keep your Oxygen levels nominal. That could go someway to help explaining the gasping.
Aquadvisor is inaccurate, and should not be relied upon. It fails to take into account swimming space, temperament incompatibilities, alongside a host of other important factors. Much better to ask here!
The reason for a larger tank being preferred when dealing with Bettas in communities (a...
Do not add any more fish to your tank, for the love of Poseidon. Your tank is too small for a Betta to coexist with small schooling fish, let alone all the others you intend to add. It wouldn't even be big enough to house any of those fish individually.