Cheapest system is a DIY set up using yeast and sugar. You can use multiple 2L bottles for each tank and rotate 1-2 out each week to maintain co2 levels. It's easy, but weekly solution changes get old pretty quick and you can never perfectly dial in your co2 levels.
You can get a pressurized system that will run both tanks, all you need is a decent quality regulator and a manifold or T fitting so you can run more than one needle valve. You will need a bigger co2 tank obviously unless you want to refill more often.
I use a 5lb tank on my 20 long and it costs around $20 to get it swapped out for a full one. The disposable co2 cartriges that you see running the smaller systems are usually MUCH more expensive, and IMO are not worth buying at all unless you're running a very small tank. If you're serious about co2 I'd say get the largest tank you can afford/have space for and go with that, and get a decent quality dual stage regulator to go with it. It should last you for many years.
As far as the skimmer goes, it should add oxygen to the water but won't add much in the way of co2. A general rule of thumb is the more surface agitation you have, the more co2 in the water will be lost to the atmosphere. A lot of people run an airstone at night, but so far in my application I haven't had a need for it.
You can get a pressurized system that will run both tanks, all you need is a decent quality regulator and a manifold or T fitting so you can run more than one needle valve. You will need a bigger co2 tank obviously unless you want to refill more often.
I use a 5lb tank on my 20 long and it costs around $20 to get it swapped out for a full one. The disposable co2 cartriges that you see running the smaller systems are usually MUCH more expensive, and IMO are not worth buying at all unless you're running a very small tank. If you're serious about co2 I'd say get the largest tank you can afford/have space for and go with that, and get a decent quality dual stage regulator to go with it. It should last you for many years.
As far as the skimmer goes, it should add oxygen to the water but won't add much in the way of co2. A general rule of thumb is the more surface agitation you have, the more co2 in the water will be lost to the atmosphere. A lot of people run an airstone at night, but so far in my application I haven't had a need for it.