Will my nitrates come back?

[Brendan Van Epps]

Hello all,

I just started an aquaponic greenhouse where I teach. I have a system at home that I have had for many years but it is in a completely controlled environment. The greenhouse is a new variable that I am still learning to work with.

My tank is completely cycled and I had a nice, high, amount of nitrates...as in enough to grow tomatoes and other fruiting plants. Unfortunately I live in Central NY where the temperatures fluctuate mercilessly. The past two weeks was 85+ and then it has since dropped to barely 60 with night time lows in the 50s.

Needless to say my fish tanks have dropped in temperature and so has the nitrate level. I know that bacteria are dependent on temperature. Will the nitrate levels jump again when the temperature (which is going 80+ this week...see I told you) in the tanks go back into the 70s?

Thanks!

[Alex Veidel]

Depending on the fish you're using, they might have an increased appetite relative to the increase in temperature.....increasing their feed rate to adjust for their appetite can sometimes cause nitrate spikes. Other than that, I don't think there will be a problem. Fish can handle quite a few nitrates.

[Rob C.]

The lower temperature lowers the metabolism of the fish which lowers their ammonia production.  So, you wind up with less ammonia to be converted to nitrites, which will be convert to nitrates.  More simply stated:  cold weather = lazy fish = less nitrates.

As long as your ammonia levels aren't rising, then your nitrifying bacteria colonies are doing well.  

If your ammonia levels are rising, but the nitrate levels aren't rising, get a nitrifying bacteria starter bottle to add to the water.  That will jump start the ammonia cycle.

Good luck!

[Brendan Van Epps]

Thanks Rob! That's what I was thinking but I wanted to be sure.

[Gilda Baise]

If you are going to grow greenhouse plants throughout the winter depending only on your fish tanks for nitrates, you will have to heat the water. Both the fish and the bacteria slow down as the water temp falls. Tilapia, of course, die when the temp gets too low, so I assume you are raising a cold tolerant fish variety, which will slow down in the amount they eat as the temp drops to 50F and stop eating altogether around 40F. This is good since they would quickly poison themselves without the bacteria to convert wastes. It does no good to add bacteria to the system if the water temp is too low. From what I have read the bacteria are only efficient above 60F. If you have warm sunny days in which the water heats up, the bacteria will become active. The problem in an unheated greenhouse is retaining water temp at night or through long periods of cloudy days. One large commercial greenhouse in a northern area piles manure around the outside of the greenhouse. For me, as a backyarder, I let the fish do their winter thing and use raised dirt beds under a hoop house for the winter, growing cold tolerant plants. I do plan on testing methods of holding heat in the water tanks, using them to moderate the temp, however, I live in mid-VA, where night temps in winter seldom dip below freezing.