Liquid carbons work very well when dosed at the levels plants need. I normally recommend 1ml liquid carbon for every 2 gallons of tank water, even more for high light tanks. If you have delicate shrimp you have to be careful when using liquid carbons as it doesn't sit well and can kill some delicate species. Also remember some plants such as Anacharis and Val's can be very sensitive to liquid carbons and can melt.
You can buy Metricide 14 Day Solution, a gallon for around $20. It is twice the strength of Excel and very economical to use.