Fish advice please?

I recently upgraded my 80 gallon fancy goldfish tank to a custom 200 gallon tank. I have
a refugium, 2 large Rena filters, a large Aquaripure nitrate filter, chiller and a lighting
system that mimics night and day lighting and I can set it for cloudy days and storms with lightning!
I love to just sit and watch my goldfish in their BIG new tank.
I have done A LOT of research on fancy goldfish and their care.
You have too many fish in a 20 gallon tank already. You are supposed to give the first goldfish 20 gallons and any additional fish 10 gallons. With just the 4 goldfish, you should have a 40 gallon
tank for them to comfortably live and grow in.
You also have to be careful housing goldfish with plecostomus. If there is not enough food for
the plecostomus, they have been known to go after goldfish and suck their protective slime covering off, which can then weaken them and makes them more susceptible to ick and other parasites. They can also attack Goldfish that have a wen ( the lumpy part on the top of their head ). I used to have a pleco a long time ago but after witnessing it chasing my goldfish around, I gave him back to the pet store.
In addition, goldfish are cold water fish, while plecos, cory cats and danios are warmer.
Fancy goldfish are VERY hard to take care of. MUCH harder than when I had a salt water tank.
With the size of your tank and with how many fish you already have in it, you should be doing
water changes every few days. I also suggest ( if you don’t have one ) to purchase a freshwater
test kit and regularly check your water quality. You would be surprised how quickly ammonia,
nitrates and nitrites can build up. If you start to notice black marks on any of your goldfish,
that is a sign of too much ammonia. It is actually a sign of them healing from the ammonia burn
they received when their tank had too much of it in it.

Through much trial and error, I have learned the hard way of how to keep fancies.

Pixie-Dust already nailed it lol. Goldfish are amazing critters, among the more difficult fish to care for PROPERLY, and probably the most abused animal on the planet- and no one cares since they can’t emote through their faces, ‘oh it’s just a fish’. Only part of hers I’d change is even bigger than a 40 for your crew- I find 3 too much for a 55.

Being commons doesn’t help- maybe makes them a little tougher, but also gives them a greater potential size.

As for the cories? Don’t do it. Even if your tank were larger, they’d just get starved out by the goldfish- they do their own bottom scavenging, as I’m sure you’ve seen. Cories aren’t poop eaters, no one is. Even if you tried to target feed them, the goldfish would probably get most of it.

Just because your goldfish are not fancies, they do still eat and poop a lot. Goldfish are little eating and pooping machines! They may be more ” hardy ” than the fancies but they still do not like high amounts of ammonia and nitrites and after repeated exposure, will wear their immunity down. A weakened immunity leaves them open for parasites and bacterial infections and can lead to death.
NO fish likes swimming around in it’s own waste.
Just because your tank looks clean, you could still have high levels of them.
I REALLY suggest getting an API freshwater test kit. It comes with PH, High PH, ammonia,
Nitrite and Nitrate testing solution. Unless you test your water, you don’t know what is going
on with it. Goldfish do prefer higher PH ( hard water ), soft water is to acidic for them. That
is why you will want to test both PH and High PH to see where your tank is at.

PH should be around 7.5 for goldfish

Ammonia should ALWAYS be 0 ppm ( No fish should be in a tank with ammonia! )

Nitrite should ALWAYS be 0 ppm

Nitrate should always be < 20 ppm ( ideal would be <=12 ppm )

Temperature for goldfish should be around 74 degrees F.

You probably already know this but make sure to always treat your water with a de chlorinator.
I really like Prime, but there are many different ones out there to choose from.

I also like to use API Quick Start after every water change.

My biggest issue is that I never quarantined new fish or plants and I am paying the price
for it now. I purchased some plants for my refugium and the person I purchased them from
promised me she would treat them for snails for me. Sure enough, I just put them in my refugium
and a few weeks later I had snails! I also purchased some show quality goldfish and thought
that since they were soooooooooo expensive they would be parasite free. I just put them in the
tank with the fish I already had and BAM all my fish got flukes and I lost several. Now I have
a 10 gallon hospital tank that EVERYTHING new will be quarantined in for 4-6 weeks + treated
for parasites and any bacterial infections. When not in use, I keep my little Betta fish in it
to keep it up and running.

I hope this helps you with having a happy and healthy goldfish tank!