Hello,
It's great to hear that you're planning a revamp for your 72 gallon Red Sea aquarium and this is important task that needs some planning.
Firstly, let's talk about the water. When you're doing a major overhaul like this, it's recommended that you save as much of the original water as you can. The water in your tank is a complex ecosystem that includes beneficial bacteria that are essential for the nitrogen cycle (the process that breaks down harmful chemicals like ammonia and nitrites). If you replace all of the water, you're essentially resetting the cycle and potentially causing a spike in these harmful chemicals.
That said, during an overhaul like this, you can aim to keep around 50-80% of the original water. Store this water in a clean, non-reactive container while you're performing the clean-up and redesign. Once you're done, you can reintroduce this water back to the tank. Remember to keep the stored water at a similar temperature to avoid shocking your fish and coral when reintroducing.
When replacing the sand, remember that your substrate also houses a significant amount of beneficial bacteria. If you're replacing all the sand, consider keeping a small portion of the old sand to mix in with the new sand. This will help seed the new substrate with beneficial bacteria. You can also consider using a bacterial supplement available in pet stores to boost the bacterial population in your new substrate.
Cleaning the rocks is also a good idea, but again remember to do it gently. You don't want to remove all the beneficial algae and bacteria that have made the rocks their home. If the rocks have a heavy build-up of unwanted substances, gently scrub them using a brush in old tank water (not tap water, as the chlorine can kill the beneficial organisms).
After you're done with the re-design, slowly reintroduce the saved water, your rocks, and your fish/coral back to the tank. Monitor your water parameters closely for the next several weeks to make sure everything is stable. You should be ready for potential water changes if your levels start to go off balance.
Lastly, remember that this is a stressful event for your tank inhabitants. Try to minimize the time they spend in temporary housing, make sure to maintain appropriate water temperature and parameters in their temporary environment, and consider using stress-reducing products available at your local pet store.
Please note that every tank is unique and results can vary, but these are general recommendations based on my experience and knowledge in the hobby and this is more on the careful very safe side. Always observe your tank and its inhabitants closely and adjust your actions accordingly.
Good luck with your aquarium redesign!
