help: losing fish, not inverts!

I have a 45-gallon tank that's been set up for a year. No live rock or coral, only a few small fish at any given time, along with some sponge crabs & assorted urchins. Well, the inverts have thrived, but I can't keep fish alive!

I use an Eheim wet/dry cannister and a CPR Bakpak skimmer, plus a powerhead for additional water movement. My ph is perfect, alkalinity is about 1.22-1.23. No ammonia, no nitrites, and nitrate varies between 20-45. I do a 50% water change every month.

I have lost every fish I've put in this tank...most rarely last 3 weeks...even hardy fish like damsels and wrasses die. Mostly, fish seem fine for 2-4 days, are heavily breathing for another day or two, then die before a week is up.

Could there be something in my water that's toxic to the fish, but not the crabs and urchins? Am I changing too much water too infrequently?

I've kept fish for 30 years, and never had this kind of problem. Any suggestions?

Thanks to anyone who can help with this mystery!

 

Do you have any rock work at all? Sand bed?


John

 

I have many pieces of dead & fake coral, so there are plenty of hiding places. Also, I have a sand substrate, purchased 'live' in bags from the pet store...it's about 2-1/2 inches deep.

 

Sorry about all the questions,but allof those things have been in the tank for the year?


John

 

....and while I'm at it, do you feed the Inverts?

John

 

All sand and rockwork have been in the tank for the entire year the tank has been set up......no new additions.

Yes, I put invert food in twice a week, but they get most of their food scavenging the sand and rocks. My urchins are doing great! If there was something bad in the water, I'd think they'd be the first to go!

 

Yeah that's what I was thinking too Also the large water changes would effect the Inverts if that's what was causing the problem

The only other thing I could think of is a simple ammonia spike. That wll happen whenever you introduce a new fish, but with a tank that has already cycled and has inhabitnts that are fed the spke would be quickly converted to Nitrite and then Nitrate.

When you added the fish was it just one at a time or a few at the same time?

John

 

When I add, it's either 1 big fish (3-4&quot, or 2-4 smaller fish, like damsels. And when I do, I check the water daily, but there are no spikes that I can detect. Usually, my water quality is bettr than the tanks I'm buying the fish from!

My Eheim wet/dry and Bakpak are both rated for tanks larger than my 45, so the water stays clean and well-circulated.

I'm hoping there's somethiung obvious I'm forgetting, because fixing the problem would be much easier!

 

Also...my temp is roughly 77-78-degrees.

 

Here is a few more...

How long did you acclimate the fish? How did you do it?
Did you get all the fish from the same place?

How often do you open the Eheim?
What do you do to it?

How is the skimmer working?