Goldfish at bottom of tank

Question
I have a 25 gal established goldfish tank that had 2 5-6" fish.  One died and the other appeared healthy.  I added two new fish - one about 5" and a smaller one.    About a week a later my original fish has taken to lying on the bottom almost motionless for long periods of time.  Another week has passed and all 3 are now lying on the bottom.  When I feed them they become active for a period of time.  On occasion they do become more active but for short periods of time.  I do not overfeed and am more prone to skip a day feeding to ensure that overfeeding is not a problem.

Ammonia and nitrite are stable. Levels of nitrate are not in the danger zone but are higher than I would like. I have done a 20% water change 4 days ago and will monitor the nitrate.  

One thing I did that concerns me is a 40% water change immediately after losing the original fish as I feared that the water condition may have been the cause of death - but I did not confirm this prior to the water change.  The 40% change may have caused the spike in Nitrate (?).  Is there something else I should be looking for.

I saw my original fish move quite rapidly the other day and really twisted its body - jumped at the surface of the water and then retreated back to its sedentary position at the bottom of the tank.

This morning the new small fish is hiding under a rock.  Prior to this morning, he has been 'pacing' an identical path back and forth along the width to the tank - almost frantically.

This may or may not be relevant, but this tank has always produced thin layer of green algae on the glass and other tank items.  I recently cleaned the glass only with a magnetic sweeper but the algae remains on the other items in the tank.

I have three part filter system - sponge, charcoal and white pellets which I do not change on a regular basis as I do not seem to have any obvious filtration problems with this tank.

Any ideas?

Answer
I can already point out one thing, the tank is too small. Please identify if they are comet/common goldfish or fancy goldfish.

A single Fancy Goldfish can live in that aquarium no more. 3/4 Fancy Goldfish need a 55 gallon aquarium.

A single comet goldfish needs a 55 gallon 20+ gallons for each additional comet/common. Several comet/common's need a 200+ aquarium.

That pacing is called chasing the glass. It is a sign of boredom. When there is an overstocked aquarium there is much less room to swim and or you've kept the decorations as the same for a very long time. It needs more space and more hiding places.

Lying at the bottom normally lethargy, caused either by bloating, exhaustion (too tall of a tank), the water being too warm, and so on.

Could you give me their diet as well?

As for the algae, algae often occurs in aquariums that have too much lighting or that is near a window. I keep all of my aquariums on a timer for exactly 7 hours a day. Leaving the tank lights on overnight is a good way to get algae.

Algae is not bad, it does produce natural oxygen in the tank, but it does take out a lot of nutrients for live plants.

- Ash