I can move the betta, ... what do I need to test the water hardness? Test kit I have only shows these:
PH: 6.6
High range PH: 7.0
Ammonia: 0.50ppm
Nitrite: 0.25ppm
Nitrate: 0ppm
You don't need to buy anything else to test the hardness, go to your local water authorities website and look for the water quality report. Most let you type in your postcode and find out what the water is like in your specific area. Read this thread, to help understand why we need numbers rather than just whether the water company says it's hard or soft
If you're not sure which numbers you need, take a screenshot, and members here can tell you what the GH and KH are for your water, and which of the fish you have can cope well in your water, and which might be better returned to the store or rehomed. I only learned about the hardness issue last week myself, and I've had a tank for a year, so don't feel bad, and try not to feel overwhelmed, it's not your fault you were given such bad advice.
https://www.fishforums.net/threads/hardness-words-vs-numbers.461996/
My goldfish is large. I’d say about 5-6 inches.... the pet store recommended him as a companion for my betta because goldfish are “peaceful and bettas don’t Really enjoy being alone”
I'm sure you've since discovered that this is nonsense of the highest order. Betta are fiercely territorial, and prefer to be alone, it's how they live in the wild. Betta are also a tropical species that like it warm, while goldfish are a coldwater fish that will struggle in the same temperature as the betta needs. They're also poop machines that require huge tanks and like the company of other goldfish, but the fact the person said bettas need company, and to keep two species with completely opposite temperature requirements together - let alone anything else - I'm banging my head on my desk for you, I really am. How willing/able are you to set up different tanks or rehome/return some of these fish? Are there any that you/your child are really attached to, and we should focus on what you need to keep those specific fish? Like the betta?
@Deanasue is our goldfish expert, she can provide a lot of guidance.
Try not to panic too much about diseases yet, most things are treatable if they turn out to have any. But for now, you have a lot going on, and an uncycled and crashing tank would kill them faster than most any disease they might have. Focus on doing those large (75% for now) water changes whenever you see ammonia or nitrite above zero, and figuring out the filter. Cartridges that you replace every month are an expensive waste, and not as good as using plain on sponges that you rinse out in tank water or clean
declorinated water, which will last for years and won't crash your cycle the way replacing cartridges can.
Then we can address which fish you can comfortably house and want to keep, and how to do that.
Then we can tackle treating any diseases.
Big deep breaths, and I'm sorry you had such a rough introduction to the hobby! It can be peaceful and relaxing, once you get over this rough start, I promise, and it isn't your fault you trusted their advice, it makes sense to trust them, they should know! But sadly, they frequently don't, and they made a lot of money off of giving you terrible advice.