What am I doing wrong with these fish?

[Porthia]

First fish keeping experience. We got a tank and treated the water and put plants in and ran it for a month before adding some cloud minnows, cories and a loach (literally just got exactly what the lady in the aquarium shop recommended that we have in the tank).

after about 5 days one of the minnows looked a bit peaky and seemed to be having trouble and was languishing at the bottom. I did a 50% water change, adding the tap conditioner and bacteria to the water before I put it in but sadly the fish died.

I took water samples to the aquarium shop and they said it was fine so it was most likely that the fish had just been shocked by the transfer.

so we pressed on. I do a 50-30% water change twice a week and sometimes do a smaller change in between to just keep oxygen levels up.

a week or so later I noticed another minnow was spending a lot of time at the surface. I did a few consecutive days of significant water changes and it seemed to recover. However fast forward a another day or so and I noticed had a bit of a bent back (see pic).

I didn’t think it had been like that before but but it seemed to be swimming and eating well so I thought I would see what happened over the next day or so. Well now it’s on the bottom of the tank and it’s in a sort of bent shape (see other pic). I’ve done another water change but the nitrates and ammonia looked ok.

all the other fish look fine and are swimming around a lot, eating etc.

i feel really terrible for these poor fish. I know I am responsible for their lives and wellbeing but feel quite out of my depth here - the aquarium shop were not terribly interested.

any ideas?? Many thanks in advance!

How many litres is your tank? That's quite a few fish in there, they can require a surprising amount of room.

Have you got a filter in there which can collect bacteria? Don't change it or rinse it often so that the tank can find it's own bacterial equilibrium.

I think it’s 30L? It’s literally the smallest starter one as it’s technically my 9 year old’s (but of course I am the one who is actually looking after the fish)

I wondered about that too so I was totally guided by the staff in the shop. They suggested 6 minnows, 3 cories and 1 loach.

I hope I haven’t consigned them to a short life of misery in a small tank!

Oh and yes there is a filter. It has a charcoal cartridge which I was told to change once a month but I haven’t changed it yet. I have just wiped off any algae when I changed the water

You're changing far too much water. It's still a young tank, our tank filter took 8 weeks to cycle and be ready for fish. If fish are gasping at the surface then that suggests a lack of oxygen and or high ammonia levels. Have you got an actual testing kit?

This is the filter we have

30l is very small. I would say a max of 5 fish for that space so yep, too many fish. I would return a few.

Fish haven’t been gasping at the surface. Just this one fish spent ages just under the surface. It was like he couldn’t sink. He was like that for a few days then seemed to recover.

oh god, so my paranoia about ammonia and nitrate could mean I’ve actually changed too much?

I do have a kit and have been checking to make sure neither get out of the “ideal” zone as per the packet instructions

It's a long time since I have kept fish but agree that it sounds like you're changing the water too much. I used to do about 10 percent a week and also do you have an oxygen pump? Also had a plec for the algae.

Oh and absolutely do NOT touch or change the filter. It is the lifeblood of the tank. Swish it gently in the tank water every so often when changing water but absolutely no squeezing!

Thank you so much for your replies I also meant to say!

Definitely no squeezing of the filter! Just wiped the bit where the water comes out of it.

should I be changing the cartridge inside it? It says monthly on the packet.

Nah. Every few months, it's its own ecosystem.

30 litres is arguably too small for any fish whatsoever I’m sorry to say. Were you adding a source of ammonia during the 4 empty weeks? If not then that was largely a waste of time and you are now doing a fish-in cycle. Sounds like terrible advice from the shop.