I'll just throw this out there for future reference... I haven't had the best of luck using a germination trey to start seeds. But don't throw it away because it does have its place! I too was using the exact same set-up and had shotty germination. What works a little better is starting your seeds in a dampened(not soaked) paper towel within a ziplock bag. Keep it in a dark and relatively warm place(on top of the refridgerator, tv set, stereo, computer tower- whatever you have thats on all the time that stays warm).
Now once you start seeing sprouts THEN put them in the cellblock germination trey with the heat mat beneath. You'll get much better germination using the paper towel method and using the cellblock until they need to be transplanted into a bigger vessel.
Since you're already up and running go on ahead and stick with what you've got. If it doesn't pan out try the above. It'll do you well.
Now once you start seeing sprouts THEN put them in the cellblock germination trey with the heat mat beneath. You'll get much better germination using the paper towel method and using the cellblock until they need to be transplanted into a bigger vessel.
Since you're already up and running go on ahead and stick with what you've got. If it doesn't pan out try the above. It'll do you well.