Sudden Nitrite spike & Torn fin on fish

[Weapon_Master]

All,

Experience a sudden nitrite spike yesterday after noticing one of my clown fish had a torn fin on his back.

Went to measure the nitrite and found it to be at 0.2 level. Nitrite has always been at 0. The last time this was measured was 5days ago.

Regular 20% water change has been done 3 days.

Most recent addition was a hammer coral 2 days back and 2 out of its 5 branches appears to be dying. Could this be the cause of the spike?

I noticed that the torn fin on the clown does not show that it is rotting and neither is it reddish in colour.

Anyone has any advice what should be done?

My Tank Details are as below.

Tank: 2.5 * 1.5 * 1.5ft

2 * True Perc Clown

1 * Purple Fire goby

1 * 2 spotted goby

1 * Scooter Blenny (Lastest addition 1 week back)

2 * Fire Shrimp

1 * Cleaner Shrimp

1 * Pistol Shrimp

2 * ###### Shrimp

Water parameters measured below

NO2: 0.2 (Bioplast Test Kit)

NO3: 0.1 (Bioplast Test Kit)

Calcium: 540 (Salifert Test Kit) (bringing this down)

DKH: 8 (Sera Test Kit)

PH: 8.0 (Bioplast Test Kit)

PO4: 0.25 (Salifert Test Kit)