I really don't know what's killing off my guppies/endlers

I have a ten gallon tank of female guppies and female endlers. It has too many fish in it but it's not really badly over stocked. I do water changes once a week usually but sometimes I don't do them that often because I'm tired. I have live plants in this tank and the filter is rated for 10 gallon, not over rated like I should have with so much livestock. I also have an air pump going for it.

I came back from a week long vacation. I cleaned the tank before I left and again when I came back. There's since been a few water changes, about one every two weeks. There's been a lot of green algae in and on this tank b/c of nutrients and because it's sitting between two windows.

So I left the aquarium covered for three days with towels. It didn't get too hot.

All of a sudden, fishing are dying at the rate of one a day. One showed dropsy but the others aren't showing any signs at all. They just turn up dead. I don't have any really new additions to this tank. I've had the female endlers for 2-3 months now. I q'd them before adding them.

I have no idea what's wrong. I think that a less than well maintained tank would suddenly be causing deaths at this rate, particularly for such hardy fish.

I don't even know what to do now. All I have not expired on hand is epsom salt but I'm thinking that it may just stress the fish and make whatever it is worse. Should I just wait until my meds get here? Hopefully on Monday?

The tank is unheated, at room temperature. My water parameters are in my sig. I can't test the water because my foot is broken and it's really hard to get around. The best I can do is clean the tank and I've been doing that...

I didn't cover up the top of the tank. I may be working without a full deck of cards but I'm not that bad. Lol. The algae didn't even die off, as far as I can tell. But I did a water change right afterwards. There's been no sign of redness around the gills or clamped fins.

I'll find a way to do the tank maintenance, I guess. I need to get a wheelchair but insurance has been impossible. Ever tried to carry things with crutches? Almost impossible. You're right I shouldn't treat without testing the water... Maybe my weakening filter (forgot to mention it) also led to this. Well... I feel dumb. Poor fish.