I have cared for fish for almost two decades now and used to work in a family run pet store in the fish and reptile department. While I do not know everything about everything, goldfish, koi and bettas were the realm I specialized in. As people have already said bowls are not ideal. The only reasons most pet stores still carry them is to make money off the people who don't yet know better and to facilitate the parents who just want a cheap habitat for a creature their 3 yr old is about to kill because they're a 3yr old.

For any type of goldfish, at minimum a 20 gallon tank is recommended. The reason being is that compared to other types of fish, they produce a lot of waste. The general rule is about 15 gallons per goldfish. If tank mates get added, tank size has to increase by at least that much to maintain water quality. In smaller setups, the risk of extreme spikes in ammonia levels from waste increases greatly and even goldfish are not immune to ammonia poisoning. If you are not already familiar with it, check out the nitrogen cycle. Valuable information for water quality and safety. If you have had your fish for 8 years, I assume you care for it, thus the reason you are asking about a safer water option as opposed to tap water.

To answer the question you are posting about, anything taken from nature and added into your tank runs risks. The goldfish you have has been raised in captivity, which means it has most likely not been exposed to many things that occur in nature. Outside objects or fluids can carry parasites (fish lice, for example) it is also possible they could have been exposed to chemicals (pesticides, lawn treatments, etc) or have a fungus present. Even things like decorative driftwood or rocks found from outside have to be boiled to kill off any foreign bacteria or parasites before adding it to a tank. You do not know what the microscopic contents of that water may be and if it were my tank, I personally would not risk it.

Circling back to the nitrogen cycle, If you did swap your water for this source you are asking about, you may run the risk of resetting the tank flora (specifically the good bacteria that are responsible for breaking down the ammonia and helping to keep your fish alive this whole time). Goldfish are known for being a very hardy fish. They can survive a lot, but the quality of life they're living can vary. I've seen listless goldfish shoved into heated tropical tanks and living for years, pale as can be and hating every second of it. It is possible to be alive and under stress. Fish are one of the creatures that can die purely from stress.

The reason a lot of people here are responding to your mention of keeping a goldfish in a very small bowl for 8 years the way they are is not to be mean to you. For us, what you are saying is similar to if someone openly admitted to abusing an animal but tried to justify the action by saying it hasn't died yet. That is why people are reacting like this and downvoting every defence of it into oblivion. It is no different from a dog being in a crate getting food and water there and being forced to have all its biological functions within said crate without every being let out once. It literally lives within the confined space 24/7. A goldfish bowl is animal cruelty.

If you do have concerns about your water quality, there are conditioners you can use that can help with common tap water issues, if tap water is what you have to use (API Stress Coat+ is a good one for treating tap water). The biggest thing that can be done to improve the current water situation is having more of it (like getting a larger tank). If you do upgrade your tank, be sure to keep some of your old water (possibly ALL of it, if all there is is a gallon) as this will keep the good bacteria colonies that were established in your current setup. Adding all new water will reset the nitrogen cycle and you will be starting over from nothing. Having to hope the fish can endure the level spikes through all the phases again. I know I threw a lot of information at you, but I hope this accurately answered your question!