Am I Doing Something Wrong?

[Novice fish owner]

would be grateful for any advice-
Tank one year old. Recently noticed small ulcer on Danio, not sure whether to take action with medication as it is still eating and behaving normally and have heard medication can cause problems with the water quality for the other fish? Also recently lost a neon tetra who gasped for air and swam near the top before it died. One snail spotted which must have grown from egg on new plant. Then had water tested at pet shop, all levels fine. Nitrate levels slighlty high but were advised not to do anything as have real plants in the tank. Since then have added the new fish as stated below and lost a neon tetra after only one day!!

tank temp:~28'c
tank size:30 litres ish

Fish Symptoms:
Danio- small red ulcers on the side of the belly
recently lost one tetra fish from first batch and one from the new additions both swam at the top of the tank the first one couldn't breath and had its mouth open a lot and the tetra that had only been in the tank for one day had a band around its belly where there was no colour and wasn't as active as previous day.

Volume and Frequency of water changes:
usually 20-30% water changes every 2 weeks

Tank inhabitants:
4xStripped Danios
3xNeon tetras
2xcatfish
5xcherry barb

Recent additions to your tank
the cherry barb and (2 surviving) neon tetras

Exposure to chemicals:
recommended dose of King British "safe water" plus tetra Aqua "Aqua safe"

Not sure what to do next, any advice welcome.

 

[WILDER]

You are severely over stocked for a 6gal tank, danio are active fish and should not be kept in anything below 20gal, to be honest none of them fish should be in that size tank all it good for is a betta.
I doubt very much that your water quality is good as you are overstocked, i would take a sample of your water to the lfs and tell them to write the readings down for you,
If a fish was gasping at the top of the tank for air it sounds like bad water quality, for the ulcer I would do an immediate water change and you will need a bacterial med.

 

[Novice fish owner]

You are severely over stocked for a 6gal tank, danio are active fish and should not be kept in anything below 20gal, to be honest none of them fish should be in that size tank all it good for is a betta.
I doubt very much that your water quality is good as you are overstocked, i would take a sample of your water to the lfs and tell them to write the readings down for you,
If a fish was gasping at the top of the tank for air it sounds like bad water quality, for the ulcer I would do an immediate water change and you will need a bacterial med.

I guessed the size last nite but measured today and its 50 litres, will try the med.

 

[WILDER]

Ok, good luck.