Did you test your water for ammonia, nitrite , pH and nitrate levels before you did that big water change?
If it were me I wouldn't treat for a fungus unless I see there is one.
As far as filtration goes, it won't weaken a fish unless it is already weakened from stress.
I think testing your water is a big clue to what is going on.
Is it a new tank recently set up?
Your fish could be sick/poisoned from new tank syndrome, high spikes of ammonia or nitrites.
Please test your water and post the test results back here.

After  reading it 500 times on THIS forum I don't see a 50% water change being considered BIG!    With  an over populated tank   I MUST do water changes more frequently to prevent my fish dieing  however it is normally only a 25% water change with a 50%   for emergency such as Kissy being sick &   if  my levels are way off.   I;m  sure they're stressed  lately with the more frequent water changes  &  mating(bullying) going on.   Can not remember  the #s from my  a.m. water change today   in the mayhem of  trying to help my Kissy  by changing water, looking online for answers & calling the Vet  I  forgot to record the parameters in my fish journal     I know the ammonia, & both the Ns were off.  If the water was normal I wouldnt have changed it       I  used the Fungus tab (1 tab=low dose) as a last resort as directed  per  Fishpert until she could check  her out  after hours.      Happy to report  it did something as she finally started swimming a little by evening       The final diagnosis was exhaustion from  high currents in the tank & breeding   & a touch of ammonia shock.  Still not sure how in the world they doctor a fish  but I feel much better now. 

Finally made the move to the 60 g. tank  tonight:)     Introduced Kissy to it 1st & let her enjoy a half hour by herself.  Silly girl didnt act sick at all ,  when the  rest started joining her she quickly reverted to her clamped bottom sitting ways.  I'm about to decide she's unsociable now.   They seem happy at the moment & are enjoying all the room to swim.  I was reminded to  put the top on when Monster  did a nice little flip.   He cracked us up.

Stating NOW before its asked to save ya'll the time  "reminding"   60 g has been cycled for over 6 weeks,  parameter reading  this evening   PH-  7, Ammonia- 0, Nitrite- 0   ,Nitrate-1, GH- 250   
Will be testing again each evening until I'm satisfied  the tank  will be good for all occupants

Thank ya'll for the concern of everyone's fishes. 

I always test my water before adding anything and before doing any water changes too.
Your test results will tell you how big of a change or if any change is necessary.

Thanks again ya'll.  Hectic day when your fish are ill.  I apologize if I came across as crass  my nerves were on end all day. Kissy is my fave sweet girl  not sure I'd enjoy my fish without her.  That's bad to say as I do love them all  her sweet smile &  bubble kiss in the morning make me have a much better day:) I'm hoping  there'll be no more water issues now that  the gang are in the proper size aquarium.     fingers crossed it all stays good in there & we can have a normal  biweekly  cleaning.  &  not have to do any major water changes.   

Yes Nossie you're correct  tank was cycled fishless   so if I understand correctly thats means I  have at least another month of  "laborious"  water  issues  as it adjusts to accommodate changes due to fish waste?    Unfortunately  I've already gotten all 5 fish in the tank together  so Kissy  can't enjoy being alone.    I think   I may  put her  in a 10g  by herself for awhile.  I just need to rehouse some guppies so I can move her in.

It can get overwhelming reading all the conflicting information online & different pet suppliers don't know which way is up  sometimes LOL.   1 thing  like about this forum  is the  knowledge base available  &  folks that  agree fish can communicate what their ailment is.    I've gotten lots of good advice &  ideas on this forum alone.   Ms. Hanna has gotten me interested in planting some live plants too.   Think I better get the hang of fishkeeping 1st though lol.

Hanna, thanks 4 calling my water pristine lol  after the trouble I've had waiting on the bigger tank to cycle its  nice to hear  just once      Maybe  we can keep it that way.    When the mansion is finally complete  I dont  believe I'll be doing anymore 50% changes that's a LOT of toting   buckets lol.   Not sure which part of filter is the media  but the new tank has a power filter with a triple action filter with carbon  & bio- fiber.   What  pet supplier recommended  for the 50 g.


Kissy update   she was  active this a.m. but  after her afternoon snack she retreated to her little hole she's "dug"  & has been sitting on the bottom since. No amount of encouragement will get her to come out either.   My  mother came by to see the new setup &  thinks she's made a nest & is laying eggs.  Not so sure about this.   

hi
my goldfish Ladybug was my favorite, she was up on top for days and started bottom sitting last night, but when others nugged her she went for a swim and eat to. but about 10 min ago she died
all my other fish look healthy and happy, she looked healthy no sign on her body. i will miss her so much

klari: I'm sorry for your loss :C Do you have a water test kit? Buy one if you don't, make sure it contains tests for pH, ammonia, nitrite and nitrate and use it whenever any of your fish are acting strange
How long have you had the tank up? How big is it, and how many fish?

Not the best pic in the world  but wanted to show how I doubled up on  stacking  lol   MTS  u know  gotta try to please the hubby some how so I put the  small guppy tank under the goldfish living room tank

I LOVE your guppy setup! Wish I could have something as colorful and cute
And your little goldies look so cute in that tank! Did you think about buying a background poster for your tank?

Thanks bunches Mindemae. U just saved me a dozen trips or more toting a bucket lol.   I'll get an extender in the a.m. already  planning on running to a fish store I just heard about   that's 3 towns over YAH  maybe we got a  good mom & pop shop  we'll see      
Nossie, haven't  thought to get a background yet.  Changing the setup  made me move to a different room  lol  the  old tank had all my ceramic dolphins  placed around it  & no where to set them in here      Better add  a poster to my list of goodies to get tomorrow.  hmm looking over there now I have  my framed dolphin scene hanging on the wall the tank's against  wonder if I moved it down if it  could serve as a nice view 4 them?   Oh Nossie I forgot to tell u  they LOVE  the bloodworms  never saw fish go so crazy over a treat b4 lol