Fish arent eating/strange behaviour

Question
HI,

I've got a 180 UK litre tank with 3 green chromis and two clowns.I have a protein skimmer, do 10-15% water change weekly with salt water from LFS. Fish an dtank been running for 18 months.
Small clown started acted strangely lying on its side almost,but swimming hard and not going anywhere always near the bottom of the tank. Yesterday I found a dead green chromis but think that was injured before but not sure how??? Now none of the fish are eating and all acting strangely. Both clowns just swimming in same spot at bottom of tank then swimming about madly for a bit. Tank levels are all normal, nitrates at about 20 which is where they always are, won't go any lower, and have a piece of polyfilter in there too.
Please help, I don't want my fish to die!!

Answer
Hi Katie,

20ppm is way too high and if it's been like that for some time, nitrites and ammonia are certainly building up slowly in the system.  I sure hope you're not using a canister filter.  They require weekly cleaning in a salt tak, not monthly or bi monthly as some recommend.  But weekly.  They are nothing but a nitrate factory...with that said, if you are at 20ppm nitrates, your nitrites are high and ammonia is probably off the charts.

When you have fishes dying, it also causes nitrites.

Injury or not, there is an urgent need to change out 25% of the water asap.

Check your salinity as wlel.

The clowns swimming in one place, and the all around lack of hunger, fishes at the top...all are very serious signs in a reef system.  There are nitrites or ammonia and the fishes are suffering.

Skim the tank heavily today.  Change 25% of the water, that's about 50L, and make sure the temp isn't high because that, right now, will ultra stress these fish out more.

I hope you write back with further questions.  :)