New tank additions, 1 angel fish dead +1 sick. Please help

[Nossy]

So my ex and i bought an established tank. Aquaone 620T. It had 2x Angelfish (paired), a Pleco and a Bristlenose Catfish. Now, i come home from work one day and my ex had purchased 6x neon tetras and a random group of 4x small plants. I had told her not buy ANYTHING for the tank until i know exactly whats compatible but we all know how often couples listen to each other. So being a friday night we couldnt return the neons and we didnt have a spare tank so we shot out and bought a wee shelter for the neons to hide in incase the Angels got really aggressive. Angel charged once or twice and then everything was fine they all went about their own agenda.

Now this is where i felt things started going astray. I replaced the old lamp that wasnt working (it had two, one of which hasnt been changed for two year but one had blown) and added the small plants. It was at this point i noticed the tank greening up and one of the plants dying. The pet shop owner had said there is two types of algae and the brown is part of it, but it is dying im 99% sure. Now the other plants and even the fake plastic plants are slowly being covered by this brown algae. The night i was planning to clean it out (it actually wasnt bad i just wanted to be sure) i came home to a dead angelfish in the corner covered in white gunk. So out he came and i done 1/3 water change and a 1/3 the next day adding de-stressing agent each time.

Since then the remaining angel isnt eating and is almost erratic during feeding time. Back and forth along the back of the tank and today is very slowly pottering around the bottom. All other fish seem to be fine. Pleco and catfish seem to be a bit more active and the neons are now over fed with bellys, woops. Water test out to be spot on with nitrates and ph's and water temp is consistent at 25 degrees.

Any help on fixing this would be great as i do in the end want a community tank, the Golden Angelfish is quite a good looking addition.

[Adrienne]

Okay, a few of questions first.

When you purchased the established tank how did you transport it home? Did you take everything out of it and put it back?

Did you rinse the filter out?

Did you bring any of the water that was already in the tank home with you?

Do you have ammonia, nitrite and nitrate test kits? I see you mention nitrate and pH but what is spot on?

Is this your first time fish keeping?

How long have you had the tank for?

Where in the country are you?

[Nossy]

Sorry about my late reply. The second angel had died in the end. Almost as though it was choking on something. Unsure what it was. No2 was 0 N03 was 5ml I from memory. So very minor. The tank was already established and seperated every during transport and re-installed. That way we could keep all of the good bacteria. It had been running for two years. The fish died after over a month I guess of transport. Since then we have lost one neon with the same type of thing. Mouth open and almost gasping. I had rinsed the filfilter out with the tank water I took out when doing a 20% water change. I'm in Timaru, South Island and first time but I do have some local help.