gold fish problem

Question
I  hope  you  can help. I  have  2 goldfish in a  10 gallon tank. they  ve been  doing great  ever since  together. lively all the  time and energetic.  yesterday night they simly started  sitting  on the  floor  all the  time. Esp.  the  one  that's a  little  bigger  than  the  other.  while  the  smaller  fish  just  sticks  around  it too.they didnt  swim  up as  they  always  do  when  i feed them,   dont  see or  react  to  my  presence.I  changed  half  the  water  and  filter,  though just  changed  it  2days  ago(i use water  conditioner adn salt as well),when  i  switched off the light  to  clean the  tank  they  started  swimming   all  around  like  nothing  happened,  but  when  the light  was  back  on they dodged  and  hid  under  the  plant  and  keep  sitting  still. Never  reacted  like  that to  light.I always  turn  off  light   around  9.30pm,  and  back  on  7am. today  in the  morning  they were swimming,  they  even  ate,  but after the  light  was  on, went  to  bottom  and  hid.Can't explain  the  weird  behaviour!!  Please, help!! thank you!!

Answer
Hmm, this is a tough one, Anna!

Lets try to dig deeper into the situation. Has anything else changed recently? Food type, filter media, water quality, temperature, tank mates? Any of these changes could invoke a fear or change in habit.

If any of these are the case (test the water to be sure!) then thats it, problem solved. If not, check the fish over for any abnormalities: lumps, missing scales, buldges, fuzzy film, spots and specs, bloody spots, torn fins, etc. If any are present, then some sorts of infection is the cause. Try changing their diet for a few days and see if that helps. thaw out some peas and cut them up and feed a few a day, skipping all other foods. The change in diet could cause the fish to perk back up as well.

Hope this helps get us started to a solution!