I have gone through this before and here is a list of things to expect.
- Your live rocks and sand will absorb an enormous amount of copper and make dosage monitoring very difficult. You will likely end up dosing 3-5 x the regular dosage before the rocks/sand saturate in copper absorption.
- Copper will kill everything except for the fish. You will have an ammonia spike. How much spike depends on how much life there are in the rocks/sand. It may or may not be a problem in a 180g. If it is a problem, you will be scrambling to change 100g of water at a time as you cannot use dechlorinators to detoxify ammonia with cupramine, so always have 100g of water on hand.
- Here is the kicker, and not something you'll want to hear. Copper is algaecide and will kill ALL of the algae. You may be thinking wow that's perfect for me, but get this: without all the algae, there is nothing to absorb the nitrate and your nitrate will easily shoot through the roof (> 100ppm). Coupled with the fact that you can't use a skimmer, you will be changing 40% of the water weekly to combat the nitrate.
- At the end of the treatment, removing all of the copper takes a LONG time. I'm talking about months, not weeks. You are literally waiting for all of the copper to leech back out of the live rocks to be absorbed by carbon or cuprisorb, so you will be leaving carbon/cuprisorb in your sump for many many months. FYI, after I finished my treatment, it took 3 months before algae started showing up in the tank again. That gives you an idea of how long it takes to completely remove the copper. Also, it was not until 6 months after the treatment ended that my nitrate started dropping. During that whole 6 months, I was doing 40% water change weekly to keep nitrate under 80ppm. For my tank, 40% is 100g. Imagine the amount of salt I had to go through during those 6 months.
So hopefully you see the negatives of dosing copper into DT. Is it worth it? Definitely not, and I will never, ever, ever do that again. I did that in my early days of fishkeeping and didn't know better, and in turn gave myself way more trouble than if i had just picked up a large QT to perform the treatment. 100g of water change per week is A LOT of work! not mentioning very expensive!