You may want to forgive me for anthropomorphising a fish, two fish actually but I suppose they are a lot less smelly than mice and this is just a behavioural study - nothing scientific.

I have been feeding them for the last nearly four years a very basic supermarket brand fish flake which they preferred over the more expensive nutrient rich pellet type food that are tiny little spheres that float on the top of the water. This with the occasional peeled smashed pea which they love.

The other week I had noticed them both very lethargic lying on the bottom of the tank. It has been very hot so I initially put it down to that. Also, this is common behaviour for the fantail (Glenda) as she has to drag around pretty much her body length in her tail whereas the smaller normal type goldfish (Golda) who is yellow rests on the bottom very rarely apart from overnight and a rest around 11.30am. So anyway the filter had somehow become unplugged. This was for about two or three days. After that time I noticed them being a bit off. Glenda mostly. She is more the 'canary' than Golda.

She has always been what some may call 'a bit sickly'. She is without a doubt the more sensitive of the pair.

So the filter being off lead to an ammonia spike which I don't think I have had to this level before. Which was likely why they were both lying at the bottom of the tank more than normal. They didn't appear distressed they were just flat.

So after turning the filter back on and leaving it for a few days I did pretty much a full tank change. New water, new same gravel, a rinse of the sponges in the old water and a wipedown of the gunk on the filter itself. I also saved a bit of the old water to kickstart the re-establishment of the bacteria required for a balanced system.

The improvement was pretty much immediate on their return to the tank. A few days later I noticed that the fantail and the small yellow fish both had what looked like a little patches of darker skin, not raised, just discolouration. I looked it up and this is often an indicator of healing ammonia burns. Likely the tank being off for a few days led to the spike. I didn't have a test kit as everything had been so settled for ages that I hadn't realised I had run out, hence the water change as a precaution.

After a few days the scabs have nearly completely gone and things are returning to normal. To assist with recovery, I thought that maybe some of the pellets I still had would be worth trying again. They are more of a complete food with what the packaging says has everything required. I decided to pre-soak them, as I do the flakes. Not soaking them leads to them gulping air after feeding which the noise annoyed me more than anything else.

I am sure this is particularly boring for anybody but it is very interesting for me so that's why I'm documenting it. As I said, Glenda has always been the more sickly one. She is also the smarter one. If an accurate cognitive indicator assessment existed for goldfish she would undoubtably score pretty high.

She reacts almost immediately and investigates new changes when introduced to the tank. She responds to interaction. She even managed to jump out of the tank on Christmas Day dislodging the corner of the tank cover enough to get out. She landed on the floor, wriggled momentarily and then lay still, I picked her up and plopped her back in and she was fine.

Also, about 2 years ago I introduced a U-shaped tunnel feature mostly for her to hide in as Golda can pester her a bit. When I introduced the tunnel she immediately investigated. Just the entry on both sides, several times and probably within an hour or two she went through one side and came out the other. She's never had a problem with it and will often meander through.

However, I have never seen Golda go through. The past week I have noticed he has started doing what Glenda did in the first few hours. He has started to go in the entry at the most half his body length. Yesterday was the first time that I have seen him go his full body length in. But he immediately turned around and came straight back out.

It is also easier for him to turn around in the tunnel as he is half the size of Glenda. It possible that's simply why Glenda just went straight through. Although it is also possible for her to turn around - I ensured this when buying the thing, maybe she just prefers not doing a 180. Who knows?

So the pellets. A total hit. It took them the first time to figure out that they could 'chew' more than one. I didn't know this but they chew with their oesophagus something that the flakes require very little of. A bit like having a multinational cheeseburger as opposed to even an old school greasy spoon version.

Their improvement was literally instant, they started flitting around the tank, both far more active and then Glenda resting less. Their excitement at feeding time now resembles a puppy that has only ever had dried dog food but is given a chop one day and consequently goes bananas whenever there is a BBQ.

I suppose Glenda is like me, she is dragging around this weight through no choice of her own and has simply learnt to adapt. Or maybe she is just one cluey fish. She is also more resilient when the environment is balanced, she's fed properly, comfortable, with no threats and no variables (like the tank filter being unplugged).

*** Just an aside - Curious that my autocomplete replaced that last sentence with "there are no friends and you are being unplugged". Who knew auto-correct's 'sense of humour' has evolved from simply replacing "ducking" or "Kind retards" to contextually based corrections? Dataset training → token prediction → contextual embedding → conceptual clustering → emergent behavior that mirrors narrative themes? Oh the wonders.***

I know this isn't ground breaking scientific understanding - it is pretty much the basics. What is new for me is knowing which dots to connect. The gravel variable dawned on me a few days later, perhaps the combination of the ammonia spike (I am sure this is not the first ammonia spike that has occurred) combined with fresh gravel that lacks any natural environmental coating was also a contributor. It can't be ruled out.

How Glenda functions when she's comfortable, fed properly and there is no threat is textbook. This also determines endurance. There is less physiological incentive for Golda, he manages quite ok at baseline. Glenda may not be the tougher of the two physiologically but is demonstrably the more adaptable.

How they have grown