Fish are dead

[Mike Roe]

I have a system the is 3' wide, 3 ' deep, and 30' long. I have Since it's not our main growing season, I only have  7 trays with plants in them and I use an eb and flow system. On for 15 off for 45. I have a second pump to aerate the water. I started it 2 months ago with 12 blue Tilapia. They are suppose to be more tolerant to the colder temperatures.  I live in Phoenix Arizona, but this time of year the water gets pretty cold. I put a 300 watt aquarium heat in it, and when it started getting cold, they seemed to gather around it, and seemed to be doing fine. We fed them pelts and occasionally greens like lettuce. They didn't seem to eat much as the water temperature dropped. We had about a day and a half of rain, and I came home today to see all of them floating. Some were barely breathing most were dead. Can't believe it's an ammonia issue, since I only have 12. So now I'm wondering what kind of fish I can put in that I can use. We use to have two ponds with gold fish and they did fine through the winter. Granted I would like to have the benefit of Tilapia to harvest them, but right now I need fish for my plants. Any ideas. Maybe Bluegill? Don't think they'll breed. Maybe I should just though a bunch of goldfish in. Sure could use some advise at this point

[TCLynx]

Well if your water is getting cold enough to kill tilapia, then most warm water fish (bluegill or channel catfish or even goldfish) will not be eating much till the water warms up so you might need to supplement to keep plants happy if you are short on nutrients.

[Richard Wyman]

If your system has been running a while it will continue to feed the plants quite a while in winter without fish.  How long is a guessing game though.

[TCLynx]

And it will depend on the plants too.  If you run out of fish and the nutrients are gone, you might just keep the bacteria alive with small doses of fish emulsion and seaweed extract which should give nutrition for your plants at the same time.

I know that with media bed systems the beds collect the solids and they slowly break down over time which really does give you a "nutrient buffer" of sorts which will keep slowly releasing nutrients for a fair bit of time especially if the winter plants are light feeders like lettuce.