If you guys say it's ok and good to do 50-70% water changes of new water.. Then what's the difference between a 3 month old tank and a 3 year old tank if at any given time only 25% of the water is new?
As far as I understand marine aquariums are more about biological filtration, so once the tank has cycled then shouldn't it be 'ready' ?
I am baffled at what the difference to 3 months and 12 months is IF people are changing 50% of the water on a regular basis.
Marine systems are all about stability of core parameters;
for example;
Heaters/ chillers to maintain temp
Fresh water auto top off systems to replace evaporation and maintain salinity
Dosing pumps to maintain trace elements
Refugiums and Deep sand beds to mitigate NO2/ NO3 and PO4
The difference between a 3 month old tank and 3 years is the maturity of the bacteria for biological filtration - same as in fresh, if you have a canister filter that is used to handling the bioload of 3 goldfish and then you throw 3 big Oscars in and a bunch of bristlenose in, the bacteria in the filter wouldn't be able to cope with the extra feed and fish waste and there will be an ammonia spike which will affect fish health and could even cause deaths. But if you add stock slowly, bacteria numbers have time to adjust accordingly.
Many people have different maintenance schedules depending on tank size, bioload etc. Some do it weekly, fortnightly, monthly even yearly - challenge is to do what works for you and your system.
The other difference with between the 2, is that the longer the NH4 and NO3 stays high, the more will be absorbed by the rock and leach back into the system no matter how many water changes you do - that is why you want undetectable parameters before any stock is added.
It is a particularly big problem with second hand systems having nutrients locked into the rock, many people go to great lengths to 'cycle' second hand rock in RO water before it even goes into the new home in order to draw out as much NO3/ PO4 as possible.
Another thing is any deviation from the norm will spark a mini- cycle ie disturbing the sand bed, moving rock around. If you are moving house then you will definitely experience a second cycle.
Don't think I am being all narky or anything but after being in the hobby for 5 - 6 years now, I have done things the hard way and blown a bit of cash and lost a few fish and corals. It is a steep learning curve going fresh - marine and am just chuckin in my 2 cents worth to help the newbies.
The old saying for marine tanks is that nothing good ever happens fast !
hope it helps.
Ryan