First Crash...

Hey all.

After a recent short holiday of over a week, leaving our nano reef up to the care of housemates, we're experiencing our first tank crash :(

Tank and livestock info:
9/10 Month old tank
23L nano with ZETlight LED.
2 Clownfish
1 Torch
2 Acans
1 Button
2 mushrooms
2 peppermint shrimp
X turbo snails
1 Narciuss snail

Parameters:
PH - 7.8
Nh3 - 0
NO2 - 0
NO3 - 0
CA - <190mg (didn't register)
KH - 6 (Pretty normal for us, been dosing KH buffer for a while)
Mg - didn't register
Phos - 0.5
SG 1.026
Temp 26C

Did a water change, continued dosing KH buffer and adding CA.
This morning torch coral has decided to shrink up almost entirely, clowns no longer able to host. And when feeding them last night they didn't eat (first time they haven't)

I'm at work so can't do anything until I get home (such a frustrating situation!) but I'm thinking I need to continue with the water changes and dosing, and tomorrow get some magnesium additive from the LFS as well as something like phos rx?

Any other advice?

Thanks.
jon.