Everything is dying, help me please ...

[ckckck]

ur tank is a disaster mandarin fish need a well matured aquarium with plenty copepods ur tank is way to small your local fish shop must be an idiot a 20 gallon tank is to small start again mate and take things slowly ive noticed lately people set up marine tanks not realising the cost and time involved keeping a reef tank healthy is a full time job and very exspensive