The water in my fish tank has turned a green color what can I do

I have had the tank for a long time and never had this problem.It's not in direct sunlight and I haven't done anything different.

When I noticed the color change I did about a 75% water change but it is still the same.

I'll give you my standard algae/green water answer in a moment. First of all, never change any more than 50% of the water at one time. You run the risk of throwing off your nitrogen cycle (and then we'll end up having problems with high ammonia and nitrites).

Now my standard answer:

I'll give you my standard algae answer:

"Have you had your nitrates tested lately? If not, take a sample to your local pet store that sells fish and get it tested. Anything less than 40ppm is safe, but you want it less than 20ppm to inhibit algae growth. If it's more than 20ppm, you need to do a water change. You should be doing at least 20% a week. If your nitrates are too high within a week's period, chances are, your tank is overstocked. Also, consider cutting down on feedings. If you're feeding your fish daily, consider feeding them every other day. Excess food will contribute to high nitrates as well.

If your tank is in direct sunlight, consider moving the tank or closing the curtains/shade. Also, leave your aquarium light off for a couple of days. Algae needs light to grow. You take away the light, you can't grow any algae.

Consider planting some live plants like java fern. They will help convert the nitrates into food for themselves.