Skimmer question
 

I have an euro reef rs 125 for my 75g tank. I drilled a hole in the collection cup to go to a 2L pop bottle. Lets say if I decide to upgrade to a 110-120g tank. Can I just put in a larger pump for the skimmer it will be able to handle a pretty heavily stocked 110-120g tank or will I need a larger skimmer altogether?

A Skimmer being a very important item in your tank, I would personally buy another one. Not only are the pumps pretty expensive for those skimmers unless you can find one used, they are designed to run with a specific pump. Tuning it would be more difficult and most likely be a head ache to deal with. I'd sell it and get a larger one.

That being said, theres no reason why you couldn't run that for a while until you saved your pennies for a new one.

+1

It's the number/size of the bubbles (in other words, the total surface area of the bubbles) and the amount of contact they have with the water that matter from a skimming perspective. Modding your skimmer with a more powerful pump would not necessarily make it a better skimmer. There's a reason why skimmers for larger tanks are also physically larger - the volume and shape of the reaction chamber is an important part of what goes in to it's capacity rating.