Fish are dying and I don't know what to do!! :(

[LAReefer4Life]

It could have been the coral you added July 7th. Anything wet added to the tank has potential to bring in disease. Sorry for your troubles! Wishing the best for your remaining fish.

 

[Jay Hemdal]

Thats no evidence.
A fresh tank doesn't have these issues, but every day your nitrosomonas and nitrobacter count and detrius increased until the bacterial load was too much for the tank and oxygen levels dropped. You even said you had increased nutrient levels as of late, which means more food for your oxygen consuming bacteria.

In my 15 years of reefing I've run all of my tanks without a skimmer and ever time the oxygen levels dropped dangerously low after some time during the night. After that i have put an areation stone inside each of them. I even borrowed a 2000 bucks expensive probe to check so i have proof.
Every one of my tanks has an areation stone with air pump set on a timer for the night.
I highly recommend to you to do that too but it's your choice.

I ruled out low dissolved oxygen/high CO2 because the corals and shrimp were fine, also the fish loss was spread out, not acute as is seen in hypoxic events.

The easiest test for this would be to measure the pH of the tank first thing in the morning, before the lights come on, and then at the end of the day. If there is a significant rise, then the tank needs more aeration. If the pH stays above 7.9, aeration isn't the issue.

Jay

 

[Jay Hemdal]

So do you think that the rescaping in the tank may not be the source, but it could be what triggered the disease? By causing stress they became susceptible for anything that was slumbering in the tank? So let's say I accept that it may be to late and I let the last 2 fish be in the tank and (maybe) they pull through, how long should I wait before I can even think about re stocking the tank? 2 months? Or should I try and catch them, put them in a separate tank so they can't infect my main display anymore and the parasites/disease cycle gets broken up? I hope you know what I mean

Re-aquascaping the tank could have been a stress event that started a cascade of issues. For example, fish normally have some Hexamita/Spironucleus protozoans in their gut. When fish are stressed, these organisms multiply, making the fish even more stressed and then sick. Sometimes, diseases like flukes can simmer along, with the fish reaching a balance w2ith the parasite. When stressed, the parasite suddenly gains the upp hand.

Jay

 

[Tavero]

I ruled out low dissolved oxygen/high CO2 because the corals and shrimp were fine, also the fish loss was spread out, not acute as is seen in hypoxic events.

The easiest test for this would be to measure the pH of the tank first thing in the morning, before the lights come on, and then at the end of the day. If there is a significant rise, then the tank needs more aeration. If the pH stays above 7.9, aeration isn't the issue.

Jay

Corals and shrimp have different oyxgen demands. Fish too. There may bei some ground dwellers that are fine while others are suffocating.

Ph isn't a good indicator for oxygen at all. My PH never dropped under 7.9 even when my oxygen was at 0.5mg/l. Alkalinity can buffer a lot of co2.


Edit:
8mg/l 0.22mmol saturated oxygen is producing co2 that is equal to a single point dHK. 1 dhk is 0.17 mmol cabonate.