Cool. I agree. Overstocking is the problem and I can see why. First of all, I'm trying to do a 120gal tank. I thought I could get a kick butt system if I put $3K into it (that's a lot of money, right?). Well, come to find out, I'll need $5K or more to set it up correctly and support the bio-load that I expect. The hobby is fun but EXPENSIVE to do it right. My guess is that people either have space constraints or fund constraints so they end up going with a smaller tank and under-sizing their filtration to save costs.
I commonly see that a 55 gallon tank is the 'minimum' recommended SW
tank size for the hobby. This tank is going to be either 36" (3 feet) or 48" (4 feet) long. That's a lot of space to take in a house. Now, figure all the equipment that you either need to mount, hide, or hang and even more space is taken. Now comes the real kicker... A 55 gallon tank can only support 3 - 4 small-ish fish without being considered overstocked.
So, you've got this huge tank that's taking a TON of room (and took a TON of money) and you only have 3 - 4 fish in it. Not very impressive, right? I would also venture a guess that anyone setting up a tank would setup filtration to support that exact size tank. So, as soon as you over-stock your tank, the filtration is overloaded and now you're into doing water changes to help the filtration out.
To further that point, and this may or may not be correct, it seems like people with larger tanks don't do as many water changes as someone with a smaller tank. The reason that people give for this is the total water volume is larger and therefor doesn't change as fast/much. What they really mean is because it's harder to overstock a larger tank, the filtration system can handle the larger bio-load. No need to do a water change as often.
I also agree that doing your water top-off's should give you enough trace elements that you don't need to do the water change for that. And if you feel like you're not getting enough trace elements, they make additives with trace elements. Plus, I just read an article on this website about RO
water and they mentioned that the salt that you use to mix to make the saltwater has additives and trace elements in it. So this will also help replace trace elements (as I believe you'll still need to mix small amounts of saltwater to keep your SG
stable, right? Maybe I'm wrong on that.. not too sure).
The bottom line, IMO
, is if your filtration is working correctly and properly sized for the bio-load that you have, then you shouldn't need to do frequent, or large water, changes.