Putting sand on the top layer of soil is supposed to prevent them from being able to lay eggs on the soil. I've heard that putting raw potatoes on the soil attract the larvae. When I had this problem with citrus I just repotted the plant to a soil mixture that drains very easily. They were all gone after that. I'm not sure if that'd be a good idea for tomatoes, though. There are tons of old threads about the same topic, so you may want to take some time reading through those.

Use a better quality, faster draining and less water retentive soil mix and don't water as heavily

BTi should be extremely effective, but it's not instant. You have to keep in mind that it's not a contact poison. It's a biological life-cycle disruptor that needs to be ingested by larvae to work. One or two applications may not do it because the BTi doesn't kill the adults, pupae, or eggs, only the larvae. You have to make sure there will still be BTi in your potting medium by the time the fungus gnats that are safe from the BTi as pupae when you first apply it emerge as adults, mate, lay eggs, and have those eggs hatch into larvae. I would use it in every watering for at least 3 weeks just to be sure. And dunks aren't the best form to use, as they're designed to release the BTi slowly into water over time. A granular BTi like Gnatrol or Knock-out Gnats will offer you more control over the quantity of BTi you're applying.

You also need to treat every plant in your house at the same time, not just the seedlings.

Life cycle of fungus gnat from https://ag.umass.edu/greenhouse-floriculture/fact-sheets/fungus-gnats-shore-flies

BTi works well in conjunction with sticky traps to cut down on the annoying adults while you wait for the life cycle to complete and all the resulting larvae to be killed.

In future, don't use potting mixes indoors which contain organic material when starting seedlings. I made that mistake once. Never again. Sand on top works better as a preventative than a corrective. But you need to water from the bottom and use a thick layer of sand, which can be hard to do with young seedlings in starter trays/pots. A thin layer or sprinkle of sand at the top does nothing.

thanks i probably gave up to soon i am going to try covering the entire box w ith a fine mesh .

They like wet soil/mixes. Make your your mixes have good drainage, add vermiculite or perlite or fine pine barks to it. Don't water too much. Allow the surface to dry out.

If the soil is infested, covering the box won't stop them. They'll just keep mating and breeding inside the mesh. Unlike something like a fruit fly, they don't need to fly to mate.

i will use new soil then cover i am desperate seems i can't beat them

A BT drench applied properly on the correct schedule should work well. If not, changing out the soil will do it, but you need to make sure you clean the soil off the roots well to remove any larvae hiding in the roots.

But, as previously mentioned, whatever you do, you have to do it simultaneously for every potted plant in the house. Otherwise, they'll just keep coming back.