ok im pretty new at this so this may be a stupid question

ive made a base for a tank using a picture frame and pollyfilla.. got the texture just right and left it overnight to dry.. this morning i woke up to find a bunch of large cracks on the surface

what did i do wrong?

Coupleof things may have come to play.
The base material could have been too thick.
The base material could have been to watery.
You didn't add any white glue to mix.

Can you simply fill in the cracks?

i didnt add white glue...what does this add to the mix?

It improves cohesion, I prefer to use plaster of paris rather than polyfilla, even though the products are very similar.

looks like ill be using a lot of mr surfacer on this one

Go to your local DIY store, and pick up a small tub of LIGHTWEIGHT SPACKLE to fill the cracks.

Try this some time. I drill holes in my base so the plaster has a stick point, also rough up the surface to help attach. When you mix your plaster, add a bit of liquid dish soap( helps to spread), and white glue ( helps hold it all together and gives plaster streangth to support trees and other add ons. Also your cheap paint. Make it darker as it will be absorbed and lighten up a lot. As far as your cracks, if you mix too dry or dry too fast it will crack. Try a wetter mix or a damp cloth over it as it dries. Check out concrete sites on a construction site, they use damp burlap to keep it plyable for the next day and avoid cracking. Just fill your cracks for now with suitable filler, no-one will ever know. Hope this helps.

thanks fellas