I am having a real problem with high phosphate and determining where it is coming from. I first had the problem when I set up the tank about 1.5 years ago. However, I found out it was from my tap water and when I switched to RO water, the problem went away.
Now the problem is back (cloudy water that creates an algal bloom if I leave the light on). The tank is not by a window, I do not overfeed (once a day), and I change the water every 2 weeks with RO water. I have added a few live plants which seems to have helped, but my phosphate levels are still very high. Where else could this be coming from?
I have a 75 gallon tank with 3 angels, 1 gourami, 4 guppies, 2 tetras, and a very small catfish. I have not changed anything in the tank in at least 8 months.
Any advice would be appreciated.

I'm having the same issue - I use RO water and dose no phosphates, but phosphate concentrations in my tanks hover around 5 ppm no matter what I do. I have heard that some phosphate kits are notoriously inaccurate, so you might want to try testing either with a different kit, or try testing with the kit you use on water you're 100% sure has NO phosphates in it.

I've never heard of phosphates causing cloudy water, however. Cynobacter outbreaks, yes. But cloudy water? Are you sure it's not green water or a bacteria bloom?

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Officially fishless until after the move....tune in later for details!

The two main sources for phosphate in a fresh water tank are rocks and added tap water. Pull out any suspect rocks. Start making all water changers (<20%) and water adds with Distilled water and your Phosphate should start dropping. Phosphate is one of the controling minerals for Algae Blooms in fresh water, the other is Nitrate. High phospate levels is also disasterous in a Coral Tank.

It starts out as cloudy but then goes green if I leave the light on. It normally only runs for about 5 hrs a day, although I have not run it any with the phosphate issue.
I only have one rock and I have had it since day one. I do however have some decorations (sunken ship, log, etc) that are made of plaster, or whatever they use. I have also had these from the start. Is it possible they may have started to leach phosphate into the water?

As far as the test kit, I have tried it on the RO water before using it, and it shows up as 0 phosphate while my tank is over 10ppm. So I think the kit is fairly accurate.

Try feeding every other day and weekly water changes.

Don't add anything to your RO water and do weekly 50% PWC's until the levels start making sense again. That would be my advise.

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Officially fishless until after the move....tune in later for details!

I will try the water changes and see what happens. Thanks all for the replies.

Have you tested your tap for phosphates? If you have, and they are zero I would stop doing 100% RO water changes. Depending on the water, RO can have all nutirents stripped. That can be good and bad. Calcium and magnesium, found in good quantities in tap water, are needed by fish and plants and without will start to show health problems. Those nutrients also provide carbonate hardness to your water or kh. This acts as buffers to maintain your ph and keep it stable. If that kh value is 0 in the RO water, your ph could be cycling up and down and causing danger to your fish.

The safe thing to do is a 50/50 RO/tap mixture. To clean of water can be bad. You can do 100%, but a product like Seachem's Equilibrium needs to be added. There are other similar products out there.

Yes, I have tested the tap and it is off the charts with phosphate. I will try the Seachem Equilibrium with my water changes. Thanks.