i know a man who kept 6 large oscars in a 55 gallon tank so he could sex them and allow them to pair up. oscars are a lagrge so. american preditors. he was able to do this with frequent large water changes. in a small tank like yours bad things like amonia spikes can happen quickly. i read an artical in one of the fish mags that said a 50% water change provides the best resusts. any thing above that is good but the benefits are much lower. on my goldfish i only want bio and mechanical filtration i do not use any carbon in any of my tanks. the good thing about the power filters is you can put any thing in the filter you can run it to remove only waste, you can put bio material in it to keep water quality high or you can use it as it comes to remove both. i used to have a 55 gallon tank and i had a large aqua clear with bio only chem stars in it. you have had your fish for 18 months so you are do ok. the one thing that i would change is garel removal. tha gravel and decorartions are for you the goldfish don't care they take up space that could be better used as extra water. with out the gravel you can keep the tank cleaner and not have to worry about about a golfish getting gravel stuck in his mouth. :goldfish if ypu feed zuccini cut the rind (spelling) so that after the fish are done the rind will lay flat and not little circles fish have been trapped in this and die. :goldfish an easy way to judge water quality is look at the tails and look for red streakes. the tail should be clean with no red.
i have two 180 gallon tanks. the cichlids live in the living room and the goldfish live in an extra bedroom i use to read in. the tanks are 6x2x2 feet. they do not seem to over power the rooms. the book room is the smallest room, but even in that room the tank does not over power the room. as far as the work that i do and the time i spend on them. once a week i clean filters and change water. the cichlid tank gets a 50% water change. the goldfish get about a 90% water change. most of this work is easy suck stuff out to my yard and add water, the total time both tanks about 3 hours. my goldfish have a bare bottom except for some dime to quarter sized stone so the food does not go to the filter. total area of stones is small plate size. the reason i cange so much goldfish water is goldfish are dirty nasty fish giving off a lot of waste. a large tank also will react more slowly than a smaller volume of water, ie temp,amonia spikes. :goldfish