Deep breaths - and a nice cup of tea.
A lot of people are asking for info which frankly is not going to make much difference right now.
Since you have already taken down the small tank give it a good clean and start as if it is a brand new tank. The info posted on the nitrogen cycle will be useful for this but its not today's priority.
Things you will need
- Dechlorinator
- Some clean new sponges - pot scourers are good as long as they have no chemicals in them, I use plain old Aldi ones
- A gravel vac - or a piece of hose.
- Bucket(s)
The tank is a bit of a mess but frankly I have seen far worse and its easily fixable with a bit of time and patience. Talk to your local fish shop and see if they are able to take any of the fish you don't want as well as the fry. As for an action plan:
- Don't worry about saving fry, big fish eat little fish and although it sounds cruel they are quite nutritious
- Start a programme of daily water changes. Use dechlorinated water that is a similar temp to the tank. Try to change at least 25% daily but more is better (75%). If I'm not mistaken that's a 25 litre jerry can in one of your pics.
- Before changing the water wipe the inside of the glass with a sponge. Don't bother about removing every spec of algae initially - you just want to make it better.
- If / once you have a gravel vac use it. There is no need to get everything in one go. Work through the tank doing a different section every day. Be careful not to kick up too much muck from the gravel while you are doing this.
- Use the water that you take out to clean your filter media, filter case, heater, rock, ornaments etc. Again no need to do it all in one go
- Don't feed more than the fish can eat in one minute. May be tough with kids but for now twice a week is plenty
- Your filter probably has some bacteria. These grow very quickly as long as there are some. The daily water changes will help lessen the effect if some of the bacteria have died off.
Just take it slow and steady and you should see a remarkable difference within a week. When I say you don't need to clean everything at once I do mean don't even try. There is a lot of muck (and possibly pathogens) lurking in there and you don't want to release it all into the tank in one go. That's why its best to do those activities immediately before or during the water changes so you immediately dilute anything that does get released.