My fish are dying one by one

[Fishofmind]

Hello, Good day everydayone
I got a 6x2x2 ft tank and it's been running for 2yrs, the water parameters are; ammonia: 0, nitrite 0 , nitrate: 30ppm, water ph is 7.2 and the water temperature is 26°c. This tank has a 80 gal sump with mechanical, chemical and biological filter medias. I do 20% water change every week. I use this tank as a growout tank for my Angelfish fry.
So here's the thing, I add some juvenile toffee parrot fish two weeks ago but after 3 days, those juvenile parrot fish are dying one by one until now. The tank has 30pcs Angelfish(small, half inch in size ) with them and I don't have an issue with those Angelfish only the parrot fish are dying. Their diet is bloodworms, I feed them once or twice a day sometimes I skip a day before feeding them and I never overfeed those guys.
What do you think is the cause of their death?
Can someone help me?
if someone experience the same situation, do you mind to share your experience to me?
Thank you so much!

 

[Rocksor]

How many did you buy and how many are left? Did you buy the fish locally or order? If you bought from a fish store, maybe you could test the water from the fish shop, including the total dissolved solid (TDS) levels using a TDS meter. Osmotic shock can kill slowly over time when moving fish from higher TDS to lower TDS too quickly.

 

[Fishofmind]

How many did you buy and how many are left? Did you buy the fish locally or order? If you bought from a fish store, maybe you could test the water from the fish shop, including the total dissolved solid (TDS) levels using a TDS meter. Osmotic shock can kill slowly over time when moving fish from higher TDS to lower TDS too quickly.

It's from a fellow hobbyist, his parrot fish has an unexpected fry. He said he's too busy to raise them so I brought them home.
He told me they're a month old and they're almost the same size as my angelfish fry
There are approx 30-50 pcs of them, around 11pcs death until now.
Didn't bother counting because they're no more than 1 inch in size

 

[Rocksor]

Does the hobbyist use the same water supply as you?

 

[Fishofmind]

Does the hobbyist use the same water supply as you?

Yes we got the same water supply, we live in the same neighborhood.
I'll try to limit my water change from 20 to 10% a week.
By the way I tried to examine one of the dead fish and I noticed that its veins is lil puffed and red in color.
I know that it's not ammonia burn, because when I test the water the ammonia result is 0

 

[Rocksor]

Did you test with an ammonia liquid test? Or test strips?

The weekly 20% water change isn't an issue. Lowering the amount of water changes will only slow down the growth of the fish. Too low of water change amount can also cause fin deformities.

How do you do water changes? How much water conditioner do you put into the new tank? Or do you dechlorinate the water in a separate container before adding to the tank?

 

[Fishofmind]

Did you test with an ammonia liquid test? Or test strips?

The weekly 20% water change isn't an issue. Lowering the amount of water changes will only slow down the growth of the fish. Too low of water change amount can also cause fin deformities.

How do you do water changes? How much water conditioner do you put into the new tank? Or do you dechlorinate the water in a separate container before adding to the tank?

I'm using API liquid test kit, I do water change by dripping method. A 20% water change usually takes up 6 to 8 hours to finish.
I don't really use water dechlorinator because our tap water chlorine level is only 0.1 ppm

Okay I won't change my waterchange routine if it isn't the issue, though I really want to know what's happening to those parrot fish, it's my first time to keep those kind of fish and I might've a phobia keeping parrot fish next time because of this

 

[Rocksor]

Any chance you can bypass the chlorine drip and use water than has been sitting out for more than 1 day with aeration and then do a water change? Does your drip system go through activated carbon? It's quite possible that the parrots are a bit more sensitive than your angels.

Is there an airstone in this tank?

 

[Fishofmind]

Any chance you can bypass the chlorine drip and use water than has been sitting out for more than 1 day with aeration and then do a water change? Does your drip system go through activated carbon? It's quite possible that the parrots are a bit more sensitive than your angels.

Is there an airstone in this tank?

Yes sure I'll do that, I got active carbon as filter media and my tank is pretty aerated.
I got a k-1 canister inside my tank and 3 big sponge filter.
Here's another thing that I noticed only the bigger ones are dying, those who are almost 1 inch in size

 

[duanes]

I consider 30ppm nitrate high, and a 20% water change per week wimpy. I do 30-40% on my system every other day.
Parrot cichlids (if you mean true parrots, Hoplarchus psittacus), come from low mineral content water with undetectable nitrate, so 30ppm for fry could be quite dangerous, and the percent die off could be normal under those conditions.
What is your friends water change schedule? Is he/she, seeing the same proportional die off?