Tropical tank, fish dying mysteriously

[drumm3]

I've never had experience with fish before and I'm new to the hobby. I bought a 9 gallon tank that has LED lights built into the lid and I also use a heater.

To start off my tanks beneficial bacteriaI put 3 comet gold fish into my tank (I realise they're not comfortable with warmer water) and after those died my local pet store said it'd be alright to use them since I'm just getting my tank set up. They died due to ph and ammonialevels which are stable now.

3 days ago I bought 12 neon tetra, 1 elephant ear betta and 1 golden sucking catfish. I had one neon die this morning which looked white and the second had a pink tail which I flushed since I didn't want to risk contaminating my school of tetra. Most recently my betta had lost some coloration on its head and not been eating. Being so weak it was dabbling around my tank and I isolated it so it could eat but I ended up putting it back since it didn't eat and then it swam too close to the filter and got stuck and then shortly after I freed the betta it died.

What am I doing wrong? I need some help urgently?

 

[mikey11]

3 days ago I bought 12 neon tetra, 1 elephant ear betta and 1 golden sucking catfish

you getting too many fish, too quickly......

with a brand new tank you should be adding fish slowly.....maybe 2-3 a week at most

and bettas and tetras dont mix very well

 

[King puff]

Plus, golden algae eaters are a form of the Chinese algae eater. These are aggressive and get up to 12 inches. As Mikey said, Bettas do not mix well with other fish. You also said that you flushed your fish. This is not a good idea due to the fact that it could end up contaminating waterways. You described pink on its tail, this could be an ammonia burn. This is problably happening due to the massive bio load that has been abruptly added to your tank. I would return the algae eater. Then, see what happens to the neons, they do not have a high bio load. Do 50 percent water changes every day and do not dose any medications.