[Allotment Jack]

Hi. Can anyone help?
This is my second season on the allotment. Last year was a good one and i had some very good crops and learned a lot however this year i seem to be off to a bad start 
I have sown some cauliflower, sunflower, tomato, cabbage parsnip, bazil, pea and lettuce seeds as have my two young sons at the end of feb in my greenhouse.
The first to show themselves were the cauliflower which grew to about 1-2" then withered away and died except one tray full out of 3. As far as i can tell. they all had compost from the same bag and were watered from one of 2 water butts. I kept them all in propergaters with the lids on so they did'nt dry out! however i have noticed  a dark green substance forming on top of the soil. Could this be some kind of algae? it did also happen last year but everything grew fine?? Apart from the cauliflower and a few pea and tomatos nothing else has grown at all. could i be over watering them or are the just to wet in propergators with lids on?
Please help. Best regards Ricky. 

[Aunt Sally]

Definitly over-watered.  That's why the surface of the soil is green.

I always prefer to water seeds and seedloings with tap water that has stood in the can to warm up a bit and lose it chlorine before use.  Water in butts has all sorts of fungi and bacteria in which can prevent the seedling surviving.

[madcat]

While my stuff is in the propogators I don't water them.  They are generally damp enough with the condensation.  Once they have germinated and have two nice seed leaves, out they come if at all possible, to try and stop the damping off in the warm wet air of the propagator.

[prakash_mib]

This is my first year in growing. I decided to take the seedlings away from propogator and place them in drip trays straghtaway when they are about an inch tall with reflector backing. this always freed my propogator for next batch and have done with a bit of success.
And as aunty said the tap water is always best for the propogator based seedlings which prevents damping off and you have to careful while watering them when they are young. You can miss once which will make them look sad but over water it and you will kill it

P.S.: Mods! this can be moved to GYO section

[Allotment Jack]

Hi. Can anyone help?
This is my second season on the allotment. Last year was a good one and i had some very good crops and learned a lot however this year i seem to be off to a bad start 
I have sown some cauliflower, sunflower, tomato, cabbage parsnip, bazil, pea and lettuce seeds as have my two young sons at the end of feb in my greenhouse.
The first to show themselves were the cauliflower which grew to about 1-2" then withered away and died except one tray full out of 3. As far as i can tell. they all had compost from the same bag and were watered from one of 2 water butts. I kept them all in propergaters with the lids on so they did'nt dry out! however i have noticed  a dark green substance forming on top of the soil. Could this be some kind of algae? it did also happen last year but everything grew fine?? Apart from the cauliflower and a few pea and tomatos nothing else has grown at all. could i be over watering them or are the just to wet in propergators with lids on?
Please help. Best regards Ricky. 

Thank you all sooo much for you help, i really appreciate it