New guppies slowly dying off
I have an aquarium that has been running for about 3 months now. Natural gravel substrate, some mopani driftwood and heavily planted 10 gallon male guppy tank with a mystery snail and shrimp. Using a sponge filter rated up to 20 gallons. Water parameters are perfect 0 ammonia, 0 nitrite, 5.0 nitrate, ph 7.6. I introduced 4 new guppies a week ago. Sunday I noticed they were a little pale but all getting along so just assumed it was stress from the new tank. Yesterday I noticed 1 was gone, presumably died overnight and was a snack for the clean up crew. Then I noticed they were all even more dull and swimming up at the top in the bubbles. I increased oxygen and there were no changes in behavior and then I lost another one later in the day. The remaining two are looking in poor shape today, even more dull and swimming at the surface. Everyone else is behaving and looking normal but I’m worried about the few other guppies, especially my sweet old rescue this whole tank was set up for. Besides them appearing dull and swimming at the top I can’t see anything else wrong with them. They were eating yesterday but completely uninterested today. Any ideas? I dimmed the lights for them too. Any shrimp/snail safe medications I can try? I don’t have a hospital tank unfortunately or I would’ve separated them all days ago.