My gold fish have been in a 30gal fresh water unheated aguarium for 3 years now with the algae controlled well. Over the past mont I have noticed a huge amount of algae forming. The glass of the aquarium was covered with it (could still see in but little hazy looking). I do have snails in the tank but this doesn't seem to help. I took a water sample in to a pet store 3 weeks ago and they said my PH was high and to do a 1/3 water change and clean the bottom and walls of the tank. I did this right after that and things seemed OK. But about a week ago I noticed the algae on the glass sides building up again. The fish appeared to be fine, very active and eating normally. Today I went to feed my fish at supper time and saw that one had died. There were no warning signs. They were all fine this morning. I am worried there is something really wrong with my aquarium now. Any suggestions on what to do? The algae seems to be slowly getting worse although the water doesn't appear cloudy.

The best way to get the algae under control is to find the cause of it.
It could be one or all of these suggestions...
Aquarium lights and direct sun light into the tank can cause algae.
Or, You could possibly have a very high nitrate level inside your tank.
Nitrates are the bi product of a beneficial bacterial colony that keeps and maintains a healthy aquarium.
This byproduct is a good food source/ fertilizer for plants and algae.
It's non toxic to fish if kept at low levels by doing regular maintenance water changes.
Even with regular water changes, nitrates, over time can reach the toxic levels and begin killing fish. One of the biggest signs of high nitrates is when green algae begin forming on the glass.

Do you happen to know the readings of your water?
Ammonia
Nitrites
Nitrates
Ph
Hardness
Alkalinity

Knowing these would be very helpful with determining the cause...

Kae

Sorry I don't know the readings of the water. I never test it myself. The last time I had it tested was when I took a sample in to a pet store. They said everything was fine except that the PH was really high. I haven't had it tested again since then. Should I be buying something to test it at home? Our aquarium is in our living room right by a window - but it has always been there and has been fine until now. The 5 remaining fish are very active and eating normally.

The 5 in 1 dip stick tests are beginning to be real popular among fish keepers. It's inexpensive and easy to use. I always recommend that fish keepers have some means of keeping an eye on the water chemistry. You can tell a lot about the environment of the fish tank with the tests.
The water chemistry may or may not have had anything to do with the death of your fish. A good sign though is that your other fish are doing good.
You can clean off the algae, make a partial water change, and use an algae destroyer product to keep the algae under control.
The algae destroyer takes several days and may need more than one dose to completely kill the algae, so cleaning it off first would be the quick fix and the algae destroyer would stop the growth from reoccurring.

Also, if you have hard tap water, making partial water changes with tap water will not lower the Ph. You would need to use distilled or drinking water.

Please let me know if you have any further questions.

Kae

I went to a pet store last week to find an algae distroyer and they said they no longer sell anything like that. They said they found it was harming the fish. So... I went back home and did a 1/3 water change. At the pet store they suggested I put in a new carbon and sponge into my filter (as it has been awhile). I cleaned the aquarium walls and put in the new water and carbon. They also suggested I reduce some of the sunlight that the aquarium was getting so we partially blocked the window so they don't get as much light. We also purchases 2 new snails (so 3 in the tank now). It has been a week and no algae on the walls. The remaining 5 fish are all doing great and very active. I plan on purchasing a test kit light you suggested to keep an eye on the levels. Thanks

That's great news! Good job at clearing the algae!
I'm glad to hear that all is going well!

Kae