Winter Growing

After some experimenting with grow bed media in the early winter, I planted my indoor crop of lettuce and rainbow chard in mid-January. After about four weeks the plants look quite healthy. All seeds germinated in the expanded clay bed, and I then transplanted a few into the gravel bed. Ive given the lettuce in the gravel bed plenty of space, and Im curious to see how differently the plants grow in the dense and spaced scenarios.
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Expanded clay bed 2/3/2011

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Expanded clay bed 2/9/2011

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Gravel bed 2/9/11

The conditions in the water are not ideal right now: while there is plenty of nitrate (plant food), the pH is about 8 and the temperature is about 62-65 ^oF.

A pH of 7 or just below is ideal in aquapaonics. Plants absorb the most nutrients in slightly acidic conditions (pH < 7)  while fish prefer it slightly basic(pH > 7). So we compromise and try to make the water pH neutral (pH 7). The nitrificaiton process (turning fish waste into plant food) naturally lowers the pH, so the fact that mine is above 7 means other factors are at work. First, my gravel has lots of limestone in it, and the calcium it contains leaches into the water and raises the pH. Surprisingly, the clay pebbles also raise the pH, even though they are billed as pH neutral. I tested them by letting some clay pebbles sit in fresh tap water (pH 7) for a week, and they raised the pH up to 8 or 9. The plants are growing well now, but Im going to experiment with adding some lemon juice to lower the pH over the next week.

As for the temperature, the nitrifying bacteria prefer water between 72-75 ^oF. At lower temperatures, their growth rate slows down significantly.  I could use water heaters, but they are very energy inefficient, so Im trying my hand under cooler temps. The goldfish and lettuce, however, love the cold water. Currently, there is plenty of nitrate in the tanks, and I will monitor the tank to see if the bacteria are able to maintain sufficient levels as the plants near harvest stage.