what's killing my fish??

About a month ago one of my small ryukin died. I went away for the weekend and had my mom feed my fish once while I was gone. Nonetheless, I came home and found my ryukin sucked into the filter. I did a water test before I went and everything looked good. I suspect she overfed them even though she claims she didn't (*considering we have a small pond of fast and always hungry comet, not fancy fan tailed goldfish like mine*)

Moving on... last week I noticed my most active and bubbly oranda was not acting like his normal self. Usually when he hears me he swims towards the top and is ready to bed fed. He kinda hung out in this low far corner behind a statue. I was away from home for a night, next day he's dead, again sucked against the filter. Again, no issues with the water. My only suspicion is that the tank is somewhat near the AC vent and I recently turned on my AC for the first time in a few months and it shot out some pieces of dirt/dust... maybe some got in the tank and since he's always up top looking for food he ingested it? Because there were some particles near the hood of the tank and on the table next to it. Or am I just crazy? All the other fish seem fine!

So I do a water change, change the filter, and eventually add a new replacement- a slightly larger oranda. He was super lively when I got him. I came home today (a day later) and he was lurking in the low far corner like my other fish a few days before his death! He didn't respond to me tapping on the glass. Do you think he was scared and just kinda froze to go unnoticed? I actually had to give him a nudge with the net. Do you think he was just hiding/ sleeping. After I nudged and fed him he was swimming quite actively. I can't figure it out! Should I be concerned or is it just a case of random unexpected fish deaths.

I have the test strips, not a liquid kit. An instant strip that measures PH, water alkalinity, nitrites, nitrates, etc. And another strip that tests ammonia which I swish around for 10 seconds. I know I need to upgrade to a higher quality testing kit but I spent way too much money trying to save my fish this summer! Anyways, everything has tested "ideal".

I have a 30 gallon tank with 2 small black moors, 2 extra small orandas, and the new guy is a pretty decent size oranda. The water is about 72-75 degrees.

The first ryukin to die was in my tank for about 2 months, and the most recent victim was in there for 3-4 months.

When I went home today, the big oranda was in the same spot in the back corner-- he literally looks like he's sitting on the gravel. I'm wondering if he gets back there behind the statue and doesn't know how to get out! He sees the little guys swim through the openings but he cant! But once I put some food in he is super lively! I'm so confused! Also he has a little white tuft on his wen... is there a way to distinguish whether this is the normal effect of wen growth versus ich or something?!

Thankfully the spot disappeared when I added some aquarium salt. I'm thinking it was just wen growth, since no other fish are showing any signs of it.

For food I switch between normal flakes and a more expensive variety of pellets, since my moors stick to the bottom of the tank and sometimes don't get enough access to the flakes.

I took a water sample to the aquarium and everything tested well. My friend came over the other day and said I'm not feeding the fish enough, and that's why my biggest fish seems most sluggish. Now he's a big old happy swimmer! When I was a kid I had fish die from overfeeding, so I guess I was too cautious/worried about that. Problem solved. Thanks for your help everyone.

REALLY?!? I never thought about that! I don't trust myself around water and electrical outlets, sooo I'll have to wait on that until I can find someone to do it for me. I'm thinking about removing one of my decorations (air bubble dragon) to give them some more room to swim around. It's starting to get brown spots on it-- not sure if that's good or bad, but will remove it just to be safe.