I would say the only limit on rock would be if it got so silly that your fish couldn’t really swim unhampered; other than that it’s a matter of personal taste. Of course you want to stay far enough away from glass to clean it.
I wondered this myself. Afterall the more rock you have the less water you have so there's the problem of too much rock restricting the ability of fish to swim (as was mentioned) but what about their ability to breathe?
Are you worried ‘too little water = too little oxygen?’ I wouldn’t really be too worried about that— unless the tank is seriously overstocked with fish.
Why buy a big system only to fill it with a couple hundred pounds of rocks which cuts way down on water volume? The whole reason anyone would buy a big system is for more water volume and better stability. Filling with loads of rock defeats the purpose..
Just be mindful of flow and detritus buildup. I wouldn't worry about it within reason. Just keep in mind that less water means that parameters can change more quickly.
But wouldn't the extra filtration of the rock make it a wash?
Extra rock doesn't always mean extra filtration. The tank can only support a nitrifying bacteria population based on available resources. Extra surface area doesn't always get colonized to a beneficial degree if the tank can't support more bacterial growth.
But using that logic, it raises the theoretical ceiling for bio load, especially for big messy fish.
I imagine the downside is less stable salinity without an ATO, less overall oxygen, possibly less total calcium and trace elements vs. a tank with less rock.
I guess the right amount is "enough".
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