I am new to the forum, so I will give you a portrait of my tank.
-set up in April this year.
-one dried log bought at aquarium store
-tank finished cycling about 6 weeks ago
-Filter: marineland emperor 400, 2 extra slots used for activated carbon
-temp kept at about 80 degrees
-perform 10 gal water change weekly with gravel vaccum and replacing with tap water treated with TopFin water conditioner
-last measurements: NH3/4:0, NO2:0, NO3:10 on 6/30
At that time I had: 2 corys, 3 zebra danios, 3 sunset platys, 4 white clouds, 2 upside down catfish, 1 pleco, 1 silver lyre molly, 1 clown loach
On 6/30 I bought 2 dwarf gouramis, 7 neon tetras, 3 silver dollars. Before adding them, I did a 10 gal water change. I also added 4 tbsp of aquarium conditioning salt per recommendation of the aquarist I bought the fish from. Overnight, one of the gouramis died. I returned it and brought home a new one, two more mollys and three more neon tetras. Since then, my fish have been dying one by one. First one of the corys, then two of the silver dollars, then one platy, then the silver lyre molly and one by one 3 of the white clouds. I have since learned that the aquarium salt was a bad idea and did a 15 gal water change on 7/2 and another 10 gal water change today on 7/6.
I don't know if some of the new fish are eating others because the cory, platy, silver dollars and the white clouds I have all found with fins chewed off and pieces of their body chewed. I don't know if that occured before or after they died. I also don't know if the addition of the salt may have slowly killed them because the silver molly that I had from before took two days to die - I could see it get more stressed, exhausted, quicker breathing, slower swimming, being picked at by the two new mollys etc. The cory however I found alive one morning with his tailfin chewed off and then he died within hours.
My question thus, is the cause for my dying fish some of the new fish or the salt?
On another note, I was wondering what recommendations you have for filter maintenance for the emperor 400. I have washed one of the two netted cartridges every week but not exchanged either of them. I also change the carbon in the two extra slots once a month.
Thank you very much for your time and help.