Hi there
Welcome to the forum! Righty I think we can answer whats going on in your tank
First thing to check do you have a filter and if so what kind?
When you say you left the water to stand for a week, this did absolutely 0 apart from waste time. The reason people would leave a tank empty is to do whats called cycle the filter. Basically put fish live in their own toilet, they poo it hits the bottom or gets sucked in to the filter and caught in the pads what ever but it stays there while it rots down. As it rots down, it releases poisons into the water. This poison is called Ammonia and can kill fish very quickly and even if they do survive after a prolonged exposure they can sometimes have lasting damage in nerve system and other issues. However there is a naturally ocurring bacteria that will develop in your filter on the sponges and other media like ceramic rings, this bacteria then turns the ammonia into Nitrite a slightly less poisonous poison but still a poison so then an other bacteria develops to turn the nitrite into nitrate which is quite harmless in low quantities. We keep the Nitrate under control by doing water changes.
So the cycle is
fish poo (and other organic material) = ammonia = nitrite = nitrate
The bacteria will only grow when there is a source of ammonia present so while you had your tank stood with no ammonia being created the bacteria would not grow. But as soon as you add fish and their poo starts to create ammonia the filter starts, but because the fish are in there and the bacteria is just starting to develop into a colony the filter bacteria cant process all the poison so it just sits in the water with the fish breathing it in.
I suspect that this is the reason you had white spot and have had fish dieing. For a filter to cycle fully for it to be considered "cycled" can take 6 weeks to 2 months or more. So I suspect you are coming towards a point where your tank will be healthy but the fish have been exposed to these high levels of toxins for 2 months or so and its just killed them off.
Do you have an API water test kit for ammonia, nitrite and nitrate? This will help you understand what kind of levels of these chemicals are in your water. They measure the water in PPM which stands for parts per million, I dont quite know how they work but if you understand the levels that is all is important. For ammonia you need 0ppm same for nitrite and nitrate needs to be below 40ppm ideally 20ppm or bellow.
To avoid putting fish through this there is an alternative way but it needs to be done with an empty tank as we use pure ammonia which you can buy from household shops like homebase if your in the UK and when this is added in the correct dosage replaces the fish poo for the tank. This method is known as fishless cycling but the way your tanks happened is called fish in cycling and it can be done 100% safley but you have to do daily water changes of around 50% of the tank. The aim of doing this is to keep the ammonia and nitrite bellow 0.25 ppm.
Now onto the fish in your tank, afraid you have a few fish in there that will outgrow your tank but not all of them, some are okay
Angel Fish - Should be okay
They need tanks that are at least 18 inches tall as they grow to about 8 inches tall so need the room
Rose tailed tetras - This is a name that could be used for quite a few fish most are suitable but to help 100% if you can find a pic of a similar one that would be fantastic
Also how many do you have?
2 Silver sharks - AKA Bala Sharks, they get to well over a foot long need to be in groups of 5 at least and are really really active and as a result need a much bigger tank. A good alternative for these fish for 4 foot tanks is the rose line shark or denison barbs again need to be in groups of at least 5 but only get to 5-6 inches rather than the 18 inches your silvers will get to and as far as looks are concerned imagine your current ones with a bright red stripe down the side.
2 Large Catfish - Again quite a broad spectrum of fish, I suspect it will be common pleco? If so same as above Im afraid there are a lot of fish that get to well over a foot in size and are really not appropriate for home aquariums how ever if it is a plec there are actually hundreds of different species of plec that grow from 2 inches to 4 foot its just unfortunate that the most commonly sold one will grow to 18 inches (very easily in any size tank)
Clown Roach - I think you will mean clown loach, almost exactly the same as the sharks and again lots of smaller alternatives such as yoyo loaches and zebra loaches.
Guppys - good fish for the tank
Mollys - as above
Pearl Guamis - I think you mean pearl gourami but yeah good fish for the tank, some people have problems with angel fish and gourami fighting but it depends on the personalities of the fish
Orange tetras. - Same as the other tetras few fish with similar names that it could be dont think they will be anything that gets massive but if you can find pics just to make sure
I know the fish you have now might not be that big right now but they will grow, some faster than others but you do have some time to enjoy them before you move them on if you wish but that tank is not really suited to live long care of some of the fish. For example fish like the clown loaches can grow to be in their 20s same for the sharks and plecs should make their teens angel fish can grow to be 7-10 years old.
All the things Ive said about are really common beginner mistakes a lot of people make them I know I did. Clown Loaches, Common Plecs and Silver Sharks are fish that are sold far far too often to people with tanks that are too small. All of those fish need tanks that are around 6 foot or bigger. It looks like it could be a rough start to the hobby but it really is worth sticking with and this forum is one of the best places to learn about fishkeeping
Hope its helped and not put you off just being honest so you can sort out the issues asap.
