Ok this tip i found in the BSGO forums bout minning roids.when you click on an asteroid you will see the numbers in the asteroid, if these numbers are the same one of two things will be there, either it is red or it has a resource(ie: tyl,tit or water).If you click on an asteroid and find two different numbers then that roid had resources in it and was popped.Go thru the asteroid field and scan everything( if you are the only one in that field then just scan every roid, don't pop any) then go back thru scanning again espiecally the red ones.Going thru it again saves you time from flying around the system to the other asteroid fields. You will notice that the red roids will have two numbers,these have not respawned yet.Watch the numbers cause they will change and when it does that roid will have a resource it now.Go thru and pop all the resourced(colored) roids.
When trying to make a water toid it is best to have someone with you,one person stay close to the toid to scan it the other goes and pops only the water roids.You'll find that planetoids usually have an asteroid field near them. These roids,when mined, will be pushed into that toid.
Now resources can jump from the roid you pop to either the planetoid or another asteroid.You can push water to try to make a water toid but remember you might push the water to another asteroid field.In that case you'll have to keep popping water roids in other asteroid fields.
If you want to see more info on this(cause its kinda hard for me to explain here) go to the BSGO forums there you can find alot of useful info.
I hope this is useful to you, good luck, good hunting have fun.