No kidding. I went 6 months without a skimmer due to cost restraints - doing 10% weekly water changes...and using de-chlorinated tap water (huge no-no!!!) and then broke out with a horrible case of hair algae. Took another 6 months of a skimmer, remote deep sand bed, GFO reactor, etc to clear it all up. If you're using good RO/DI water, you should be ok for a while with good husbandry elsewhere without a skimmer. Don't recommend it, but it's possible. Just keep your bio-load down (don't have a ton of fish and feed heavy...and avoid non-photosynthetic corals as they require heavy feeding as well).
In the design of your sump, I'd look into adding a section for a deep sand bed (think like 6-10"). That will help as a nitrate sink later on.