Lost a lot of fish lately

I've never posted before, but I have consulted your forums before for info. I've a 55g tank that is roughly 5 years old. I have some fish that have been with us since we set up the tank. However, over the last few weeks we've lost a number of fish. We lost an entire school (5) of green barbs, 2 clown loaches, 2 kuhli loaches, 1 swordtail and a couple of neons. Now this wasn't all at once, just one-by-one over the last month. Some of the fish were quite old and their time simply may have been up, but we've never lost this many so close together before. In addition, we recently restocked the neon school, and 4 have already disappeared - no bodies found - within 2 days.

Currently the tank contains 1 blue gourami, 4 head-tailight tetras, 2 glass tetras, 2 danios, 4 neons, 3 clown loaches and 1 pleco. The water levels seem fine, save that the nitrates are a bit high, but I can't seem to bring them down and my understanding is that this won't kill off fish, although it may stress new fish. I do 10-15g water changes about once a month.

I'm at a loss. One of the clown loaches was really chasing the barbs, but it seems unlikely that they were doing any real damage. There is no sign of disease. Any help would be most appreciated. Thanks.

Thanks for the advice

Both the ammonia and nitrites are testing at 0, so I don't think a spike is the cause. Plus, I would think that would wipe out more than one fish at a time. This hasn't been a mass killing, but a slow trickle. And my oldest fish seem unaffected. The Ph is about 7.3, Nitrates about 50-60. Basically, everything is falling within the "normal" ranges according to the tests I have.

I did a major water change a few days ago - roughly half the tank. I basically do quarter tank water changes once a month, but it sounds like others think I need to do more than that, so I'll give it a try. I'll also search OTS on this forum and see if that sheds any light on it. The recent disappearance of the neons made me think someone might be eating them, but for the life of me, I can't think of who in the tank would be inclined to do such a thing. Most aren't big enough to eat a neon, even a small one.

Thanks for the help.

Temperature has fluctuated a bit

It's been beastly hot here (DC) so there has been some fluctuation, although not major. Just the basic change of season temperature balance issues. And I've never been able to get the nitrates entirely under control.

I've been doing the water changes once a month since I set up the tank, according to books and other aquarium owners' advice. We've had good luck up until now, but I will start changing more frequently in an effort to avoid this problem again.

I've always had good luck with the clown loaches - one in there now is going on 5 years old - but I didn't realize that stunting their growth also shortens their lives. I'll have to set up something different for them.

I doresearch all of the fish before purchasing them to make certain everyone will be happy and healthy. I've always considered myself a dilligent fish parent, but apparently I've still a lot to learn

Thanks again for all of your help.