Pic above !

I am always a bit angry when I see shops selling the wrong stuff - a 25 liter tank is nice to grow some mini shrimps or the like, but it surely isn't the best "beginner solution": The less water you have, the easier is it to get it wrong - the ph level or anything else. I had an 80 liter tank with a decent external pump and the thing is absolutely foolproof - clean it every few weeks and you're fine. I never even tested the water after the initial set up.

With yours, I'd recommend to be on the "better safe than sorry" side: Get fish that are known to be on the more robust side as you will likely have fluctuations in the water quality. And to be honest: Go for some cheaper ones as you are likely to, well, collect some experience...

Since you have a heater: Guppies (a group of females with one male...), neons and platy are the typical candidates for a tank as small as yours.

The problem you're having is the build up of toxins in the water from the fish. Filters need time to build up enough bacteria to deal with fish waste, and they have to have some fish waste to encourage the bacteria to grow - simply leaving the filter running with clean water will do nothing unfortunately.

The 2 viable options are to add ammonia, and frequently test the water until the bacteria are extablished, or add fish very slowly and frequently test the water until the bacteria are extablished

It's not your fault - it's extremely rare to find a pet shop that will tell you this, as they make their money from selling you replacement fish and water treatment remedies (which do very little!). Sad but true

As you have added several fish at once to a small tank, the toxins built up far too quickly for enough bacteria to colonise your filter, and so you're fish are being poisoned.

What you can do:
1. Change 75% of the water immediately. Make sure the water you add is declorinated, and matches the temperature as closely as possible.
2. Change 50% again tomorrow, and the day after, and the day after that.
3. Buy yourself a home text kit asap. Liquid ones are best, and are much cheaper on ebay than in pet stores. It MUST test for Ammonia, nitrites, and nitrates (in descending order of toxicity).
4. Test your water every day, and change 25-50% of the water each day until you get acceptable test results.
5. Going forwards, only add 1 or 2 small fish at a time, and only when your water quality is good.

(fyi, fish will tolerate changes in pH, hardness, tempetature much better than changes in ammonia, nitrite. Manor wasn't testing the important stuff for you!)

In the end, the people at fault are Manor for not giving you the information to really take care of your fish. Ignore their advice going forwards, and do your own research. I can't post links, but search for "First tank guide" and you'll find a great beginners site.

Good luck!

Argh, apologies for the double post, but I can't edit my posts yet to correct things

What I wanted to add - it'll only take a couple of weeks of frequent water changes to get everything under control, so don't be too put off.

Also, once you got the hang of it, as your tank is small, it'll only take a few minutes a day. And you don't even need declorinator! Just have a large old jug to take out old water, and get a cheap bucket for the new - refill the bucket each day after changing the water, and leave it to stand beside a radiator. By the time you go back to change the water a day later the chlorine will naturally have gone from the water, and the water should be a close temperature (at least mine is!). Then you can just pour it straight in to the tank!

Once things have stabilised you'll find maintenance is pretty low - check the water once a week for toxins, and change water based on that.

(I've got a smallish tank (about 50ltrs) and only have to change the water every 2 weeks or so, though I often do it more frequently as my fish seem to really like frequent fresh water)

Wow. Ours was also 80chf from Manor as I said above, but it's 48L.

We're having complaints from the neighbours (up to 2 floors below!!) because of the humming of the pump though. We've had it 2 months, and they are only complaining now.
We will try to put a "mousse phonique" (acoustic foam) 5cm thick under the aquarium or the furniture.

Any other advice on the noise issue? it's really about the vibrations being transmitted to the wood floor and making a bass noise for the neighbours (we can't hear it in the next room, so must be travelling vertically!)

You should have some foam under the tank anyway. I don't believe for one second they can hear it unless it's shaking itself to bits.

Regarding the shrimp a bamboo shrimp is a filter feeder and does not take solid foods. It needs to be able to sit on the current and the tank needs to be 'dirty' enough to have sufficient nutrients in the water. Chances are it was already starving when you bought it.

We do have a 5mm foam under the tank, but it's to even out the weight from what I understood... And I can testify that when placing my hand on the furniture on which the aquarium stands, I can feel the vibrations. It's definitely not shaking itself to bits though.

It is an old building from beginning of the 1900s so walls and floors are thin... But 2 floors down! I was surprised...

Hopefully with 5cm foam, it should do the trick?!

Nah.... They will find something else to complain about. I very much doubt they can hear it two floors down. Most probably its just an excuse to complain. You can put a mattress there if you want. Wont make any difference

It is a 1 year old post, but it raised a point about the topic of filter noise with external filters vs internal filters: I always believed that external filter are more noisy. The water "buffered" the minor vibrations of internal filters.
The internal filters should only make noise if something vibrates unexpectedly, mechanical part of the magnet, a problem with the small propeller.
Here I am assuming that no air pump is needed if the flow of the filter is good enough for oxygenation.

no air pumps are needed for aquriums.... air pumps actually add virtually zero oxygen to the water as the oxygen exchange occurs at the water surface.

all you need to do is ensure there is some water surface disturbance/ripple.