Wow. One or 2 times per month I find myself shaking my head in frustration at how difficult it is for a newcomer to this hobby to find quality advice. I am very glad you stumbled upon this site and asked these questions today, because you have some things completely messed up. Let me be the first to invite anyone from PetCo or Petland, who wishes to debate this topic publicly, to visit this thread.-)
Lets start with some basic concepts. In saltwater the long term goal is low Nitrate and stable alkalintiy, calcium, and pH readings. It does not matter what you intend to keep in the aquarium, be it corals, fish, or inverts, these parameters will impact the overall health of your system and have to be monitored for any sustained level of success. When selecting filtration, you must ask yourself how the filter you are using will impact these test results. The same concept applies to the depth of sand and design of your live rock structure. (pictures would be helpful)
Lets start with what you've done well. You chose to use live rock and sand. Live rock is fabulous for the overall stability of the tank. It introduces copepods and amphipods to the tank, which are great natural food sources, and help to seed the sand to become "live". The live rock also processes waste efficiently, breaking down organic acids into nitrogen gas, which leave the system naturally. The sand bed compliments the live rock, and provided you have between 4'' and 6'' depth, will effectively provide denitrification benefits to keep Nitrates low over the long term.
You also choose to use a protein skimmer, which is an important decision. The SeaClone models are adequate for a 29 gallon tank. I've used them myself on many 29 gallon tank applications. The skimmer will remove organic waste directly from the water, reducing the waste which is biologically processed by the live rock and sand. This helps to keep carbonates in your buffer system, as less acids are utilized biologically. These carbonates will help to keep calcium levels up, keeping alkinility more stable, and fixing pH at a level that is needed for marine fish. It all starts with the skimmer and the removal of organic acids.
Now, for the list of mistakes. There are a few, so lets discuss.
First, why did you decide to maintain a salinity at a level that does not naturally occur? 1.021 to 1.022 is not even close to natural seawater. Any claims you might have heard about helping to fight off parasites are misguided, as it has been shown that hyposalinity requires a salinity no higher than 1.016 (some say 1.013). You need to raise your display tank to 1.024 to 1.025 for long term health.
Lets discuss your Penguin 350. This is a very effective biological filter. Unfortunately this does not consider the type of biological filtration being provided. The penguin units, as do most freshwater biological filters, creates a high oxygen environment to grow bacteria. These bacteria break down ammonia and nitrite, producing Nitrate as the end result. This is exactly opposite of what you want in a marine system. This filter is designed to INTENTIONALLY pump high volumns of NItrate into the aquarium. Intentionally????????? The concept makes no sense at all and has absolutely no use on a marine aquarium. You want low levels of nitrate, which are achieved by the natural system of live rock, live sand, and protein skimming that you have already created. You need to remove the Penguin 350 immiately. Now. Like right this second. Stop reading and go take this filter off of your tank.-)
Ok, now that you have that taken care of, lets talk about water changes. What are you trying to accomplish with the water changes? Again, this is a freshwater concept. Freshwater systems are designed to intentionally reduce carbonate and add Nitrates. We don't have this problem in a properly set up marine aquarium, so have very little need for water changes. We do small water changes monthly, say 10%, but this is only to replace trace elements that can not be tested with test kits. There are approxomately 178 of these trace elements and fish and corals tend to respond well to small monthly water changes. Frequent changes are unnecessary and only disturb the stable environment you've worked hard to create.
And finally, lets discuss livestock. You have a small tank. This requires fish that reach a small ADULT size. Most marine fish reach 75% of their adult size within the first year of their life. Without proper space to grow they will not develop properly. Their immune systems are compromised and they generally do not fight off infections well. You need to buy fish that stay 3'' or less in size. You should be thinking 3 to 5 fish in total. The Clownfish are a good choice. The Tang is a horrible choice. You should consider a small Goby and Flasher Wrasse to complete the setup.