How often?
How much to replace?
what to treat the new water with?

Regards tinca:)

How often and how much depends on your setup on how many fish you drop.

Having a small overstocked aquarium might mean large regular water changes, maybe even more than once a week. On the other hand, you may have a large understocked aquarium, you may not have to the water change every week.

When your tank is eventually cycled, you can start working out how often the water will need changing. The best way to do this is leave the tank for a few days and then do water tests. Your ammonia nitrite should always be zero, it will be the nitrate you will be looking at. If you get to the end of the week and your nitrate levels are above the recommended level, then a water changes are definitely needed. If they are still very low, you don't necessarily have to do a water change. What you've got to remember it's not just nitrate levels that you need to check, pH is also something you need to keep an eye on. Ph will change, it could go higher, or it could go lower. If you find it changing drastically, and getting to the point where it is outside the recommended reading, a water change will be necessary to bring it back into line again. Even though it is possible to leave the tank for more than a week without changing the water, you do have to bear in mind that Oscars are very messy fish. For this reason, you will still have to give the substrate a regular clean. Having good filtration will make your life a lot easier, especially if you've got good mechanical filtration which will remove solid waste.

My personal opinion is small weekly water changes make life a lot easier. I don't do we do water changes on my 300 gallon tank, probably every three weeks. However, not all of the fish are fully grown yet, so tank maintenance may change in the future

And...would you agree that the more wood/rocks you have where debri can accumulate the more water changes or cleaning you need to do since these would be what I would call dead spots. Would you also agree that even with the best of filtration you will still have dead spots especially if it is well decorated with wood/rocks and plants? Or, should I not worry about these areas and if so so how often would you suggest to get these areas? Or, will the biological eventually eat these areas up? Which I doubt. Thank you!

You are absolutely correct, Rock/wood ornaments may well make your tank attractive, on the down side, debris will collect underneath. This can be fish waste, or uneaten food, especially chewed up pellets. How often you clean the tank does depend on various factors such as how many fish, how big your aquarium is, what sort of filtration you have got. Uneaten food and fish waste will only increase the chances of poor water quality so it's a good idea to remove it when it builds up. If you monitor your water conditions, you will get an idea of how quickly nitrate levels rise, water changes can be carried out accordingly then. To make life easier, spend a little extra money and get really good filtration, this will remove most of the waste thus making tank maintenance not so much of a chore

Overfeeding will increase the need for water changes. Once you oscars get to 3 or 4 inches only feed what they will eat in 2 or 3 mins and no more no much how much they beg for food