I have two and they completely ignore each other. I think they appreciate stimuli like plants, gravel, pebbles more than other fish (except when breedings im sure)

The life expectancy of a colony of ammonia and nitrite eating bacteria once a filter stops working. Always wondered this for emergency situations.

So any plastic jerry can made for human consumption will be fine?

I have bought a new nitrate remover and want to do several hundered litres of water at a time and save the rest in plastic jerry cans. Odd question, but how long does water keep? Lol.

Also can anyone link suitable containers?

Could be a sign of old age, most fancy guppies do not life long, and show signs of "age" after 12-18 months. They live frenetic lives, constantly harassing each other. Don't seem to cause each other damage too much though. Obviously the more they have to swim against the current in the tank the more wear and tear they will get.

How to make them grow big and fast?

1. Lots of water volume- 100 litres per fish
2. Keep temperature around 20-23 deg c
3. Feed high quality high protein foods as well as other green leafy veg etc (plenty been written about this)
4. Well filtered tank with nitrates kept as near to 0 as possible
5. Plants in the tanks for them to feed on.

Where they are raised in the far east, they are kept in large warm shallow ponds where they eat constantly on natural foods and they grow extremely quickly.

Provided you use a reputable ebay seller it should be fine. After all my goldfish came by courier so snails should be perfectly ok. Livestock get shipped all over the world after all.

I feed mine saki hikari sinking pellet as there staple food. They also get bloodworm.

Other foods to try-

Stick on glass foods- these keep them occupied for ages, make sure there really stuck on though or they get eaten in one lol.

Plec wafers- hours of fun chasing these round, although my oranda can swallow them whole now and just sit chomping on it.

Veggies in a clip on the glass

Blanch them for a minute in the microwave to soften. Brocolli, spinach, Peas, garlic etc.

I had an eheim 200, but it go noisey after about 2 year which was disappointing, then bought a fluval one... which tbh is total garbage. Given up with air pumps now and have a smaller filter running with a venturi nozzle.

Despite there looks and name, Angels are ambush predators so as said shouldn't be kept with anything bite size.

Did you advise your tank size to the aquatic store? You really need to return the angels asap.