Dirty Water?

[caoder]

I'm wondering if a tank's water hasn't been changed, just topped off for 6 months straight, what the result would be?

My in-law's tank is a 55g corner with a eihm canister filter on it. with 3 comet goldfish about 4" long and a 6-7" pleco, small bubble eyed goldfish, a small danio and a molly. he hasn't maintained the tank in 6+ months other than the topping off of water. these fish are living in it OK, but the larger goldfish have noticeable missing scales and the pleco is peeling. I know these are signs of unhealthy fish but what i can't understand is how can this tank even maintain any life at all. 6+ months of no water changes and no real plants should make for a TON of nitrates which even for goldfish is deadly. but they seem to survive. What are everyone's thoughts about the cycling and chemistry behind this is?
please don't say stuff about they need a waterchange NOW or w/e because i change my tank's water everyweek and have tried to ask him to change his water telling him the chemistry behind it with no avail. I would just like to know what is the biological/ chemical reason for this water being able to sustain any life >.>;;

 

[lipadj46]

nitrates are just not that toxic until they get to very very high levels.

 

[Inglorious]

Unfortunately situations like this are rather common and there's not much you can do about it besides try to educate people. The fish definitely sound stressed and will probably have much shorter lives than they could. The nitrates may not be at toxic levels yet but are in the warning zone, especially since goldfish and plecos are both messy species. It sounds like your father in law is just one of those people who buys cheaper fish so they can be easily replaced when they die regularly.