Too Small of A QT?

[Averhoeven]

I just had another random death in the QT tank and was trying to figure out it. I've got a 20L I've been using as a QT, but I'm beginning to think it's too small and I'll grab a 40 breeder when petco starts its $1/gallon tomorrow. I currently have 2 banggai cardinals, 2 clowns, a royal gramma, 2 small striped shrimp gobies and a purple tang. There was also a purple Firefish who was doing fine yesterday morning and eating, but I found him dead last night. None have shown any evidence of disease and have been in copper for a week now and eating well. I've been feeding mysis and emeralds entree soaked in garlic and secondaries per day (morning and night) and usually they eat it all up in a quick period.
However, last night after I found the Firefish dead, I noticed the ammonia sorry badge had a slight green tinge and I tested the water. Nitrates, nitrites, and ammonia all present. Ammonia was 0.25ish, nitrite 0.25ish and nitrate probably nearing 20. I was so disappointed in myself and upset that I let it get there. I ended up doing a huge water change and matched the copper content in the incoming water so I didn't have to restart QT (is that a valid strategy?).
At that kind of rate of waste accumulation, there's no way I can get through 30 days of copper without a water change though. Am I feeding too much? Too many fish? I worry that even with a 40 breeder it won't add a ton of filtration surface area and therefore bite change much.
Equipment :
Aquaclear 70 Hob (carbon removed)
100w eheim Jager
A marine pure sphere in the filter chamber
added Biospira
Pvc pipes
Hydor koralia

I've been out of the game for a long time (15 years) and am trying to restart everything the right way, but so far QT has been really disheartening.

 

[tgrick]

I love my 20L for quarantine. The levels you post "should" not be killing fish. Is the water cloudy? How do you check copper levels? Copper Safe has an issue with the label and the dosage might be too strong. Hanna has a new High Range copper checker that is the fish worlds BEST FRIEND. If you do not have one, save your fishes and yourself from grief and death and go get one. I'm doing this for 40 years now and THAT copper tester truly is one of the best items I have ever had. They are just recently out so you may not have heard about it.

When I put my fish in Hospital/QT I also add an air stone to the HOB filter. I use 10 gallons a lot and the 20L has never let me down. Let's start with this information 1st and go from there...

 

[tgrick]

One more quick thought.

You do have places for fish to hide...right? Having fish in the open is a big stressor and having PVC pipe elbows and tubes will really help reduce stress that might otherwise kill them. Let us know...

 

[Averhoeven]

Yup, multiple sizes of Pvc too since there are different sized fish.
I actually do have an airstone, forgot to list it earlier.
I also have the Hannah Cooper checker and have been using it. Been trying to keep my copper levels between 1.6 and 1.7 (it was 1.78 yesterday morning).