is it possible to set a potion effect on a certain area in a map like a lake to give poison or something(not a regular patter like not a square or circle)
Yes it is possible with MCEdit.
What you need is a CreateSpawners filter and the ModifySpawnerProperties filter from SethBling. http://sethbling.com/downloads/mcedit-filters/
Watch some of his YouTube videos to know how to use it.
Basically, you throw the splash potion of the effect you want in game.
Go to MCEdit and create the spawner
Modify its properties to the parameters you want
Then you could potentially fill the place with tripwire/pressure plates hooked up to a command black with the /effect command. It's not very practical though TBH
Edit: A much more practical way would be to make a command block with /testfor in it testing for an area which you want. That command block would connect to one with the /effect command.
replace x with the radius you want the effect to be applied in and replace the other stuff with what you want the potion effect to be. duration doesn't really matter as long as its longer than what you have on your clock. You can't make it work in an irregular shape unless you use a bunch of command blocks though.
What you need is a CreateSpawners filter and the ModifySpawnerProperties filter from SethBling. http://sethbling.com/downloads/mcedit-filters/
Watch some of his YouTube videos to know how to use it.
Basically, you throw the splash potion of the effect you want in game.
Go to MCEdit and create the spawner
Modify its properties to the parameters you want
Edit: A much more practical way would be to make a command block with /testfor in it testing for an area which you want. That command block would connect to one with the /effect command.
/effect @a[r=x] potion_id duration potion_lvl
replace x with the radius you want the effect to be applied in and replace the other stuff with what you want the potion effect to be. duration doesn't really matter as long as its longer than what you have on your clock. You can't make it work in an irregular shape unless you use a bunch of command blocks though.