Self sustaining FOWLR tank?

So i set my grandma up with a 150 gallon over a year ago (actually before I got my tank) and it is FOWLR. No sump only a dt. But I'd say it only has 40 pounds of rock in it. A few fish I can name, I think there is about 8.

Niger trigger
Snowflake eel
Dog puffer
Fox face lo
Tiger harlequin
Mandarin
Gold spot sandy sifting goby


So I was talking to her the other day and asked her how much money she's spent on the 2 canister filters... Apparently she doesn't service them... So I rush over there and.. All the fish are extremely healthy. I forgot to test in my amazement but before I took action I thought I'd post here, I didn't want to touch the canister filters just yet as I believe it would cause a complete system crash. She tops off with prime treated tap water that's like 80TDS iirc and to be honest I think the prime is what's been saving the tank.

The canister filters have not been touched in over or close to a year.

What does everyone recommend we do from here? I thought about leaving it as I warned her and told her what to do and do not want to be responsible for anything and seeing as how the fish are healthy however the good husbandry in me says to clean canister filters, run carbon, large water change. I definitely will not be touching it unless we get some opinions that it won't crash.

 

You mention canister filters ? So I presume 2 ?
I would shut one down and rinse it in tank water and put it back together I wouldn't do the other ne for a couple of weeks, then I would do them every month, alternating them over 2 months

 

Yep two filters! That is good logic.

 

+1 on doing them one at a time. Anything you do to the water is best done slowly as Im sure you know. Clean one, then a water 10% water change in a week or so. I would do the other in a filter in a month or so. The more trips you get to make to Grandma's house the better, fish aside. : )

 

If all the fish are healthy, I wouldn't touch anything before testing the water.

JM2C