Algae bloom or ....?

Which parameter was near zero? Did you check your phosphate levels? Leaving the lights off for a day or two wouldn't hurt.

is it a reef or fish tank are you using a skimmer

5 nitrates is plenty for an algae bloom.

You can get an algae bloom if you test at 0 for that matter. The algae can absorb the nitrates before they have a chance to register on your test.

its normal to get an algae bloom in a new tank, usually get a lot of it on rock and sand. I would do a small water change and check your skimmer to make sure its not leaking the skimmate back into the water. If thats not it then do a few small water changes a week and this will eventualy remove all the nutrients in the water that algae thrive on. Consider using a different test kit as well, salifert is a good known brand. I have used the API and salifert together to get some way different readings.

sweeTang21;1517036; said:

its normal to get an algae bloom in a new tank, usually get a lot of it on rock and sand. I would do a small water change and check your skimmer to make sure its not leaking the skimmate back into the water. If thats not it then do a few small water changes a week and this will eventualy remove all the nutrients in the water that algae thrive on. Consider using a different test kit as well, salifert is a good known brand. I have used the API and salifert together to get some way different readings.

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I'm having that issue in my new tank right now. All parameters TEST perfect, but I'm getting cyanobacteria on the sand. I was getting it on the rocks but it's gone now. Spotty on the sand now where before it was completely covered.

Saltwater tanks are kinda weird. Everything can test good but your tank is still establishing. It will take awhile, but eventually by doing some water changes and just waiting and having patience it will go away.

i cant say ive had any bad luck with them. I dont use them either but a have found a lot of difference with API, salifert and even seachems test kit.

As far as cyano goes, good water flow in the tank is a good way to prevent a buildup of it and also keeping the nutrients down. Algae consume nutrients almost as soon as they enter your tank, so they may never register on any test kit. The best thing to do is monitor the water going into the tank, watch your feeding and making sure that food and what not does not settle on the sand, so it can break down and start the build up of cyano and algae.

I've heard about the newer salifert kits testing off, but I'd honestly go with that over API.

If anything API nitrate test seems to test way lower than the parameters actually are, so if it says your nitrates are 5 they may actually be 25 or higher. My salifert kit, if anything, is saying they're higher than they are (my reef tank is at 0, but I have another tank too), which is better than fooling you into believing they're low.

My API calcium kit tested WAY high. It was saying my calcium was significantly over 500 which didn't even make sense, when my seachem kit was saying around 400 which seems more accurate.

Seachem seems to have pretty good kits too. I would stay away from API though on a reef tank.