Dead Bristlenose pleco

[mpdanio]

Hi I'm new to fish keeping. Had a few black Cory that died when I first started a few months back. They died with white fluff on them. Then all fine for the last few month. Then all of a sudden my opaline gourami died (no fluff) and today my very young bristlenose pleco died with white stuff on him. (Pics attached) I did a pH test and it's 8. Ammonia and nitrate look all ok. Any thoughts? I just put in some melafix to ensure no other fish die. I have a 200l tank and to a 60-70% water change every 3-4 weeks. Any way to stop this going forward or what may be wrong?Any insight would be highly appreciated. Thanks

 

[Calv_28]

I would do a 25% water change once a week as opposed to 60-70% monthly!

 

[mpdanio]

I would do a 25% water change once a week as opposed to 60-70% monthly!

Thank you

 

[Potluck]

okay here's one thing melafix even though most say it doesn't work it does but if you don't do frequent water changes it builds up in the tank and can kill them. It's not going to prevent deaths either its an antiseptic,
did you cycle the tank before adding the fish?
what test kit are you using?
What size tank?
What kind of filter/brand?
Water temp?
You said ammonia and nitrate..... Nitrate isn't the bad stuff is nitrite with an I and what is okay? You should have 0 ammonia 0 nitr(i)te
5-30 ppm nitr(a)te
What do you use to condition the water with and do you temperature match it when adding it back into the tank? meaning if the tank is 78 the conditioned water (seachem prime is what I recommend) the new conditioned water should match the tank..

In other words theres a lot of stuff you need to tell us so we can properly help you.