I'll second the recommendation from Carey on the eBay unit - I have that exact unit too and find it works great for my 20g. Doesn't take too long to fill a 5g bucket which is more than enough for a 25% weekly water change on a 20g. Free, fast delivery, and the 100GPD unit should allow you to expand to a larger tank without immediately needing to upgrade the RO/DI unit too.