New Fish Owner Needing Help

[Gripper]

The media won't fit, so do I need them both running for 6 weeks

 

[daizeUK]

As long as there is surface agitation then the fish will have enough oxygen.  If the filter outflow ripples the surface of the water then an air stone is not necessary.
 
So most of the fish that died were guppies plus one unknown fish and you currently have 1 algae eater and 1 guppy remaining?
 
Three guppies does sound like a good recommendation for your 25L tank.  A pH of 7.4 is certainly not too high as guppies prefer higher pH!  It might be useful to know what the algae eater and the other fish were, if you can describe them or provide photos.

 

[Gripper]

The other fish was a glass catfish, and the algae eater that attaches to the side of the tank will take a pic of that one