Mature tank and fish loss

[Lee belk]

Hi I've had a tank running for at least 3 years with a few tangs,shrimp and a pompom crab my tangs got wiped out by velvet last week so I've been feeding small amounts of food to shrimp and my crab I have been dosing atm outbreak to clean it up and its done an amazing job of it the skimmer has pulled mud out of my tank lately im worried about my tank uncycling and crashing because of less live stock I have coral as well any advise would be appreciated as I have never been down this path

 

[HuduVudu]

Are you watching the nitrite?

 

[Lee belk]

Yes I have been testing since I lost them i have done 125litre water change and got them from 50 to 25 on salifert test kit ill test again when I get home from work

 

[Lee belk]

Still in-between 25-50 on nitrate test sorry I've not been keeping an eye on nitrite I will buy one tomorrow and test that as well