I've had good luck with liquid Spinosad in the spray bottle. It kills the larvae, but I don't know which insect I was fighting only that it worked.

Supposedly a dish with cider vinegar and a drop of soap will attract and drown them. But look for other problems, male mealy bugs are gnats, some root eating larva have gnat adults. Another thing I heard was a layer of sand like mulch. Good luck. Maybe someone else has found a cure.

With seedlings, fungus gnats often are a symptom of over-watering. They also are more common when seedlings are being grown in an area with high humidity and poor air circulation. They're also attracted to soil-less potting mixes that have a high content of organic matter. To combat them, I don't water the seedlings very often. I don't let them wilt, but I definitely let the soil get about as dry as I can without the seedlings wilting. I also open windows and run a fan in the room where the seed-starting shelf is located, and do the same once the seedlings are outside in the greenhouse.

It may be that the fungus gnats you're seeing did come with whatever potting soil you're using to raise your seedlings, but I've never seen that here. When I have fungus gnats I know that it is my fault for over-watering my seedlings, or for forgetting to open a couple of windows for good air circulation or for forgetting to run a fan in the room. The role that over-watering plays cannot be emphasized strongly enough as the gnats feed on fungi that grows in moist growing medium/soil-less mixes.

One problem with fungus gnats is that it can be hard to get rid of them once you have them and they can reproduce rapidly. You can treat the soil with any product containing Bt 'israelensis', which is the active ingredient in the products: "Mosquito Bits" and "Gnatrol". I just take one of the mosquito dunks I have for our ponds and creeks (when they have water in them), break them up with my hands, and put some of the crushed dunk into water. I let it sit there for a while, stirring it occasionally, and then when the plants are dry, I water with the Mosquito Bits/water combination. You also can use Mosquito Dunks or Mosquito Bits in your rain barrels to keep mosquitoes from growing in the water.

I rarely see fungus gnats inside, but they can be a persistent pest in the greenhouse in March and April when the plants are stuck in a humid greenhouse on rainy days when the humidity is extra high. I can combat it to some extent by opening both doors and all 4 vents. The easiest way, though, is just to carry them outside after the rain stops falling and let them get some great air movement and sunshine.

I will watch the watering more closely for sure. I have had gnats in this house since I moved here. They go away for a while but then come back. I thought it was a septic problem. My sister has the same thing and she has no plants in the house.
I said in the original post seed mix but meant seed starting mix, I had 3 bins full of bags of seed starting mix and a few organic potting soil. when I opened one of the bins inside it was full of gnats. I hung a fly strip which got a lot of them but not all. I have to go to Amarillo tomorrow so I will see if I can find mosquito dunks. I need to have those on hand for rain barrels too.

Other than that the seedlings look great. I am going to pot up the rest of the tomatoes today. I get so excited to get stuff in the ground but I have to hold off on the town property just a little longer. Since its in kind of a low spot I am concerned it might frost there easier.