I very much doubt your filter is the problem as long as the flow is low or you have a sponge filter.
If he was sickly to begin with then he will probably always be skittish and sensitive.
Very true but this doesn't sound like PH shock to me, PH Shock lasts 15mins, after that its kh shock and thats what it sounds like in this situation. KH Shock can last days even weeks. You use indian almond leafs all the time?? Eaither use them or dont! The thing is it don't change your PH of the water and never will, but it will change the KH of the water up to around 3 degrees i have tested with my breeding setup compared to my housing setups that don't have almond leaves in them. Almond leaf water is expensive to do. I really mean this, most people just put the leaf in the tank and do normal water changes the thing is you can't do just normal changes. This is how I keep my water, I do in bulk so numbers are different than what you would do but you can estimate. This is only if you are going to use the almond leaf method.
Prepare a rubbermaid container with water
Sandwich bag half full of leafs, and half full of water,
Let those sit overnight. In the morning take that
water and put it into your rubbermaid container and
also leafs to. Let it sit for a few days then do a water
change with that water instead of pure water from tap
you will notice a difference. Once you do this the first time
on to second phase of the setps, take the leafs and put them
in a pot. Now boil them on simmer for about 4 hours with water
take whatever amount needed to make the tank look the colour
it is and pour the boiling water into the water that has been
sitting overnight in the rubber maid. To keep up with wate changes
just repeat the above steps till every last bit of the leafs are boiled out
then buy new leafs
Reason why you had this happen this is a guess. But you use to use normal water. The lady from the lfs told you to use almond leafs okay thats cool nothing wrong with that. So you go home and put a few in the tank now the tanins are leaking into the tank so the water is changing slowly, the KH is changing ever so slowly. Your fish is being acclamated to its new water over a period of 3 days if you do water changes every 3 days. Its now acclimated to the water its in. Now you do a water change and alot more than 10% of the water. taking water out raises ammo lvls when doing water changes that high of volume, also changes the chemistry of the water you now have in the tank. You now add tap water, by now should be a big no no if using almond water. Cause now you take the kh and put a huge spike in it so instead of the fish acclimating to the water chemistry it now just got pounced with sudden changed of KH.... I would almost gaurentee that is what happen. When I take a betta from normal tap water to a breeding setup sometimes I run into the same issue. (Stress) worst stress of them all KH stress.
I have run into this with my planted tank it having different ph values all the time is normal for a fish to stress 15 mins when putting them in, but really ph shock don't kill fish or really stress them out that much. My planted tank changes its ph on a daily bases morning reads 7 at the end of the day reads 6.5 or lower, slow increase they have plenty of time to adjust to it. But take a new fish and put them in the ph doesn't shock them its the difference in KH compared to tap water. Have you ever tested water at a lfs? Most the time you will find its tap water or tests the same reason is they do 90% water changes daily, not including bettas fish never have time to have a shock period thats why stores quartine fish for a week to get them ready for massive waterchanges.
here is a simple test take some java moss we will use this as an example and put it into a jar this will replicate a underwater gravel system that are in all local fish stores. Now fill the container with water. Test the water its the same as tap water right? Now in a week time test the water so you give a weeks time for it to cycle. Its different in its test resaults. Okay now the fun part. Do daily water changes to that jar now 90% tap water. Test the water before the water change and after the water change and record your findings. now after 2 weeks, lets go shopping for a fish for this jar for now. How about a baby feeder fish, guppy! Put it into the jar. I figure its cheap to buy a feed like 10 cents. now keeping this fish in that jar, do daily maintence of nothing more than just changing the water 90% every day. What are your resaults?
I already know the resaults are.
Problem is when your water has Chlormine in it, not Chlorine, the Chlormine because this has ammonia in it very little might register on kits as .25ppm or less, your local fish store has the same water as your tap, the undergravel filters helps to catch the waste built up right after a water change.
I wright essays more than a paragraph lol but hopefully this helps
