I’m new to the keeping fish, I’ve had my four guppies in this 6gallon tank for maybe two months now. Everything looked great and it was fine and then all the sudden it went green and I could barely see the fish in it. It’s just the water, like no algae on the side or anything. So I changed part of the water for a day or two and it didn’t help so I took a sample to the fish store by me and they said all my levels were a little high and asked if I had lost a fish (I haven’t) and said that maybe it’s an algae bloom or something. she told me to put API Stress Zyme+ and a different water softener than the one I have in it and see if it gets better after a few days. Well it hasn’t so I took it back today and they sold me yet another water treatment an API ALGAEFIX and asked if I had changed the cartridge in the filter and when I said I hadn’t they recommended I did. Anyway so now I’m home and I just switched out half the water and switched the filter cartridge and added the treatment to the water and I start googling and I find that switching the filter cartridge can be harmful and spike my amonia levels or whatever’s not supposed to happen and maybe I didn’t even need the treatments?
So I’m on here looking for advice from people that aren’t just trying to sell me stuff from their own store but more importantly trying to find the best advice to make sure I’m not hurting my fish
Green water is usually an algae bloom. It comes from a combination of too much light and too much nutrients. Algae likes nitrates and/or ammonium and in a tank so new, the chances are you had one or both of them. Reduce the light or hours you have the light one. Make sure you don't have bright light coming through a window as that can also help algae grow. Be careful on overfeeding as that will also cause excess nutrients in the water that can feed algae.
Now for the " not trying to sell you stuff" part of the reply.
If you do not have your own test kit(s), never accept the answer " The levels are fine" or " The levels are high." or " Everything is in range." etc. You want to know the actual numbers. The actual numbers tell you ( and us) whether everything is actually fine all based on the fish you are keeping. There is no universal reading that is fine for all species of fish past 0 ammonia and 0 nitrites. Some fish like higher pH, some lower pH, some higher hardness, some lower, some need nitrate levels close to zero while others can handle rather high nitrate levels. So you see, there is no universal " Fine" answer that works for everything.
At 2 months old, your tank most likely is not "cycled". Without knowing your water's parameters, it's hard to say whether your shop sold you a bill of goods because
IF your pH is under 6.8, the ammonia in your tank is naturally converted to ammonium which is not toxic to the fish. If the shop used say, the API brand test kit, it does not differentiate between ammonia and ammonium which is why you need to know your pH to determine what you have. The downside to that lower pH however is that it will take a much longer period of time for the tank to fully cycle. If the pH falls too low, it will never cycle as the nitrifying microbes will die off. So there needs to be a balance. Guppies prefer a mildly hard, higher pH water and that is the right range for nitrifying microbes to do their jobs.
I will say that your shop did you no favor about replacing the filter cartridge. The nitrifying microbes are found in areas in the tank with the higher oxygen levels and your filter usually has the highest levels. The filter manufacturers like selling more filter cartridges but until yours is falling apart, there is no need. Just periodically or when you see an accumulation of dirt on it, lightly rinse the old cartridge the the water you are changing in your tank before you throw it out so that the cartridge doesn't get clogged up with gunk. The carbon in them barely holds up a month on average so don't fall for the sale's pitch that you need carbon filtration. Water changes can do the same thing.
So for your original issue, reduce the light and get some test result numbers and we can go from there.
The 2 brands I usually recommend are API's liquid Master Kit and Fritz Aquatics liquid master kit. These will have pH ( High and low range), ammonia, nitrite and nitrate. These are the biggies. 
haven’t gotten around to getting a test kit yet but it’s still in my list of things to do.
