1st Fish And 1 Death (danios). Worried For The Others.

HI all, I need some advice from some experienced fishkeepers. My new cycled tank was ready last week for fish. Friday I bought 2 glofish danios (my son's pick) to start off the tank. Also, the LFS guy convinced me to get a BN Pleco , which I did, but have since returned because I've learned it will be probably be a problem for our tank. The danios seemed fine but it was apparent that "green" was chasing or bullying "red" a lot. I had planned on getting more eventually, but being a new tank thought starting small was the right approach. On saturday eve "red" was not looking so hot, staying near the surface in a corner, but not gasping or anything. All water parameters tested good. I feared he was stressed and made decision to add more to the school to decrease the bullying. Sunday added 3 more danios. Red still hanging on, staying near surface constantly and keeping distant. I knew something was not right and feared maybe an injury from the bully but couldn't see anything. Monday evening tried to get a close look at him. Now, I noticed something wrong with his mouth/face, not sure if it looked cottony, but something not right. I had to go out for a while but thought I should remove him from the tank when I got back. When I came home, he was dead. I'm afraid this was columnaris or something else. I'm new at this, so not sure. I didn't see any physical signs at first. The other four are active in the tank this morning, still chasing, sometimes nipping, etc. One went off by itself for a while near surface and I am anxious now. I feel like I want to add 2 more to the school so they are less stressed/bullied however if there is a sickness now in my tank I am not sure what to do!! Should I just watch and wait the 4, should I treat the tank with something? I am afraid to add meds to my new tank (live plants, too) when I'm not even sure what is happening. Thanks

 

Watch and wait for now.

 

I think that might be fungal, but I'm not sure. I have Danio Glofish and one died, but it was because it was too small, and got stuck on the filter. If I were you I would wait.
Just for reassurance I had a betta get fungal, (Or something) And he died in a different tank, but he was with my 2 danio glofish all day long, and 2 weeks later they're still fine. (I have baby fry) So I would just watch them.
And I got a new red one, it has been fine.

 

What size tank? Do you know your water parameters?

 

Hi, well I just checked on them and one is not so good. It has been staying on it's own most of the day and now it looks like it's body wants to float up towards the surface, but then it swims a bit to come back down. If one of the others chases it, it can swim OK, but then goes back to the same area and does the same thing, not swimming much. Like the other that died, I don't see anything obvious on it's body externally. But, I am new to this and these guys are pretty small. However, the other 3 are very active and busy still.

It is 20 gallon and I've been checking the water with the API master liquid kit the whole time. The ammonia and nitrites are both still 0, nitrates were 10ppm , ph7.6, temp is 76. I have not used anything like water additives to it, the only thing is the water conditioner to remove chlorine. So I feel those numbers should be correct.

 

You might want to get all purpose water conditioner, (from amazon)

 

Update: 3 of the remaining danios seem fine, but the questionable fourth one is still hanging somewhat near surface in the same area. Today he does seem a bit more active than yesterday, but still looks like something is wrong. I have looked closely at this little guy and can't see any visible signs of anything- no cottony substance, no spots of any kind, no gasping. Just staying mostly near same area near top of tank, not interacting much, not much effort swimming. I am worried that it might have whatever killed the first one, and could pass on to the others. Water parameters all good today. Wish I knew what this could be. Since he's not declining too much yet maybe there is hope but I'm not even sure what I would dose with. Any ideas?