Grrr, arrr

I had my fish tank's water tested yesterday and it's still showing nitrates through the roof. Two fish are still alive, including (surprisingly) one that is pregnant. The pet store suggested doing a 20% water change and cutting back to feeding every other day. I went to another pet store to get a no-mouth-suction vacuum (I hate my old one; it's just not good to suck on something that's had dirty fish water in it) and they offered to check with an actual kit rather than test strips. I also bought a no-scratch scrubby pad at the grocery store (rather than the more expensive pads at the pet stores) and scrubbed off the algae that was occluding most of the tank. It's still there, but it's not on the sides or front. There's an algae shelf in the back that the fish like to hide under, so I left that. I vacuumed the gravel (having a devil of a time getting it started, even so), removing the desired amount of water, filled it up with treated tap water (which I had also had my tap water tested, and there are no nitrates in it, just a tiny bit of chlorine, and the treatment takes care of that), put some more bacteria in, and we'll see what happens from there. The fish are more active, so maybe that will help. They suggested waiting a week or two and retesting before adding more fish. I still don't know why this has been so difficult; I've kept aquaria for over twenty years and have never had this sort of problem. Maybe I've lost my touch. I used to sometimes go for long periods without water changes and still had a healthy tank, with a nice established bunch of bacteria, I guess. I'm wondering if the pregnant one will have her babies soon (she has the signs), and if they'll survive in a high-nitrate environment. Keep your fingers crossed for me. Thanks.