I have considered this for a bit. I too agree now that going from a 20g right up to a 6000g, if you are going to be doing it yourself, isnt a good idea. Its simply the scope of the project, and how your own methods and experiences will change as you go larger and larger.
Methods change when you go from a 20g, to even a 50-60g, and then to a 100-200g, and then again in the 250-500g range... and again on up. My 'methods' for how I treated things when I started reefing were alot different than now. I preferred halide only systems at one time, now I prefer halide + T5s. I preferred beckett/ETSS style skimmers at one time, now I prefer recirculating needlewheels. I used to prefer closed loops... now I prefer prop-driven pumps like my Tunzes. I used to prefer a huge sump, and a refugium as large as the main tank (or at least 1/2), but not so much anymore. I used to use a remote DSB... not any more. I tried bare-bottom, DSB... and have now arrived at a shallow sand bed to be my preferred method. I used to run a tank thinking I needed tons of light beating down on the corals for best growth. Now I lean more on strong flow (90x turnover) and maintaining great water quality. I know I dont feed nearly as much as I shoul either (yet, the tank is only 4 months old still). I learned things as I expanded, and the roles of various pieces of equipment changed as I moved from one tank to the next. Im sure that in a couple years when I move up again from 125g to 500g... I will have a huge learning curve again, and my opinions and methods will again change to suit the tank. Livestock along... I once used to have a full range of hermits and snails to service a tank.... emeralds, sally light foots, etc... Now, I rely on snails alone (without hermits to kill them, they actually maintain their numbers very well), and tangs. On a smaller tank, tangs are not possible. I have found fairy wrasses to be valued not only for their color and compatibility of numbers more than 6-lines... but their flawworm and red-bug chomping habits can clear a tank of these pests in record time compared to anything else. I used to love porcelain crabs and sexy shrimp in my nanos... and I miss them... but they would simply get lost now in my tank... so I will still keep a desktop nano for their keep should I want them again. I used to love fire and peppermint shrimp... and now, I dont even bother. Unless I have a huge rock-anemone outbreak, all they will serve is to hide under the rocks all day, and fire shrimp, as cool looking as they are, and robust as they are compared to the other types of shrimp, are just too shy in a reef to see most of the time... so what is the point. Skunk cleaners are the only shrimp worth the money it seems, and even at that... they come with their own complications to the point that I would rather keep neon gobies for 'fish-cleaning', since even skunks are easy prey should a basslet or wrasse decide they are a better meal than barber. Eliminating shrimp (or just sticking to skunks which most fish seem to know not to eat compared to others) allows you to keep many fish that would otherwise eat them eventually. Hawkfish, Yellow Candy Hogfish, Triggers, etc... now you can keep them. Let me tell you... a gopack put a dozen peppermints in his 320g recently, thinking all would be well... until his pink-tail trigger just started mowing them down... so much for that! When I go larger, I will most likely skip shrimp&crabs all together (I pretty much have already except for the skunk cleaners... if they get chomped, thats it though). I will use more tangs... and larger ones like a sohal or clown, or desjardinis besides my existing ones (RSPT, YT, Striated, hippo). I mean... you have a 20g now, but how do you even know what fish you will really want to keep in such a large tank when you have no experience with their genuses even? A 20g cant hold angelfish, tangs, butterflies, rabbitfish, anthias, schools of wrasses, etc... and these are the fish you will more than likely want to keep in a larger tank. A 20 is more of a tank for a blenny maybe, gobies, small shrimp, damsels, small clowns, etc. You know what I mean? I would suggest going from a 20 to a 200-300g... thats a good step up. Its a push, but do-able. Then, jump up to the mammoth. You will have a better idea what you are getting into, and chances are your goals will change for the better as you grow. I thought I would want anthias as I went larger... boy, was I wrong. I didnt even bother looking into fairy wrasses as a better alternative, but now I prefer them easily over anthias, JMO. I just think back to when I was newer, and what my 'dream tank' would have been, and then what my 'dream tank' is now, and its night and day. Im glad I couldnt/didnt take that jump back then, and Im taking smaller steps on the way up. I also know now that even though I thought a mega-tank of over 1000g would be pretty cool at one time, now I can cap myself at 500g because thats the largest I even WANT to bother keeping up. I spend enough time every week now keeping things up... scrape algae every few days, check over the fish, check the CO2 bottle/reactor pH, check the ATO/water level, check alk/Ca/trates/phos, empty skimmer/clean cup & neck, replace the phosguard/ban and carbon, do a water change, move a coral to a better spot, clean the tunzes, trim the chaeto, dose the everclear daily, dose the potassium, feed the fish again, trim corals and frag, check temp, check water clarity, check salinity...
These things add up. Many things dont change (or shouldnt) when you go from a 20g to something larger... but the scope still does. If I have to correct my alkalinity manually because the reactor isnt balancing out just right one week, it takes a tablespoon of buffer and some RO water with a 20g... with a 125... it can take 6-10. Water changes with a 20g are simple... use a 5g bucket. With 125g on up... you need a trash can at least, if not a storage tank to make sure you have enough on-hand at any given time... just in case. 20g doesnt need a QT so much... but my 125g gets a 40B QT tank because I dont want some mongrel fish messing up my nice fish in the main display now that they are getting bigger and more expensive. Ripping apart the main tank to get a sick fish out is not an option with a large tank anymore... so it, like so many other things, need to be well planned. These are things you just dont get to know when you maintain a 20g.
