Problem: fish are dying - those that are alive are gasping for air.

4 large (4]-5") 10-year old Silver Dollars (they're probably older - I've had them 10 years). One died this morning, one is very close to dead.
1 clown loach - gasping. his partner he's had for the last 10 years died last week and was buried at sea.
2 baby Oscars - about 3"each - luckily they are doing just great - eating, wiggling, seem happy. I want to figure out what is going wrong before they get sick too!
Tank: 55 gal. Upgrading to 180 in Dec/Jan. I know the 55 isn't big enough for the O's as they get bigger.
Filter: new Eheim pro-III (2080). Moves 450 gallons an hour. It has not cycled yet, so I'm watching water carefully. Just set it up on Monday. In the Eheim pro is several layers of mechanical and biological media along with some "Chemi Pure Elite" - no carbon.
Ammonia: 0. Ever since the filter change, my daily ammonia readings have been zero.
Nitrate: hasn't gone over 5.
Ph 6.5
Nitrite: has spiked to 80. Did a 20% water change
Just replaced the air pump today - our old one was getting tired. Have one bubble wall along the back of the tank, and one in the front corner.
No sign of spots, film, or other physical issues on the fish - just the gasping

I've never experienced this before. What am I doing wrong? What can I be doing to keep my Oscars from facing the same doom the Silver Dollars are going through? Maybe it's just that my SDs are old and it's just their "time"?

thank you so much

I haven't looked into the lifespan of SD's but 10 years seems reasonable.
But the gasping is unusual.
Do you still have your previous filter running to help establish a BB colony in your new filter?
If not using some primer and adding Bacteria with the WC may help.
I'd say keep up with daily wc about 25% but some of the experts will jump in and help soon!

EDIT: Nitrites at 80 you say?
Bigger water changes may be needed to get that in check!!!!
But I can't think of any reason why it spiked except for the new filter running.

btw 10 years+ is expected lifespan of most SD's but I read about some 30 year old ones.

I didn't run the filters in tandem - I did a "cold turkey" change. There wasn't enough physical room / holes in the top to allow for hooking up the new tubes for the canister plus the old "HOB" i had before:( as a side note, the new canister (eheim pro III) was a nightmare to get setup - but wow, once it's running it is so quiet - and it sure moves the water!

what else would cause this horrendous gasping?

I edited my last post, and I think your tank is not cycled or running a minicycle....
Your nitrites must be lowered and fast!
I can only say daily WC with primer and added Bacteria, and keep testing until your filter is cycled.
Mind you it will take about 40 days.
Good luck

you could add aquarium salt to help prevent nitrite poisoning while you cycle.. 1 tablespoon per 10gal..

Beneficial bacteria may speed up the cycle, but Noddy is correct, you can expect daily water changes .

when you switch the filtering system, you should have used all available
media from the old unit to the new unit. switching cold turkey causes nitrite
spike; thus, the gasping... the fishes are not used to that high nitrite

i am not sure about chemi pure elite and how it work in detoxifying the
toxin in water.

fully grown fishes most of the time are susceptible to sudden change of
environment..much more on water parameters.

thanks for all the advice. even though I've been keeping fish for over 10 years, it's amazing how much there still is to learn! Several have mentioned adding bacteria - how is that done? My old filter is gone, so I can't get any bacteria there. I will keep testing daily and changing water as required. I guess the bright side is when I move to the 180 gallon, the filter will be cycled and ready to go:0)

This is a beneficial bacteria product from API
aquariumpharm.com/Products/Product.aspx?ProductID=44

thanks for the link for Stress Zyme - I will head to the store tomorrow and purchase! I didn't know such a product was available - and here I thought I'd have to add yogurt or something to get bacteria - LOL

No, fish will die from yoghurt or milk.
Heard stories of a single glass of milk in a 180Gl, and it eradicated all life in that tank!
Good to see you here on the site, You'll learn something new everyday!
So do I!
When your ready to run your 180 you only have to transfer the cycled filter and if you keep your bioload at the same level, you'll be ready to go !

haha

Your most welcome , I used stress zyme during my cycle and it knocked off two weeks early, its a great product I was very supised. Directions are on the bottle and easily followed.

Your tank isnt cycled because you put a new filter on and removed the old one. Check the home page on this site about cycling. Add some ammonia detoxifier like prime and daily water changes of 25%. You will know when you tank is cycled as ammonia and nitrite will be zero and nitrate will rise. Do not clean your filter until your filter is cycled. Buy yourself a test kit (the bottle types) and test every 2 or 3 days until your tank is cycled and then weekly after that