I am really scratching my head on this one since I have 20+ years experience keeping fish, but I thought I would throw it out there. I built a 125 gallon setup to be a large, planted tank which would house nothing but guppies and maybe a handful of cories (which I do not have yet) . My plan was to breed the guppies in tanks in my basement and populate the main display tank from fish from the colonies down there. I have some nice (read: Expensive ) strains going down there and a system where I move fry from the tank with the adults through a series of grow out tanks until I can ultimately sex them and separate the sexes so I can ultimately only breed the best ones. I am able to move fish around freely between all of those tanks without any difficulty. Yet EVERY time I attempt to move a fish from the breeding tanks to the main display, they all die. Not right away, they usually settle in fine and hang around a week or two but in no more than 2 weeks, they're gone. I've attempted to move fish of all ages. The first time it happened, I had removed an adult male from my colony. He lasted maybe 10 days - looked fine right up until the day I found him dead. I figured maybe he was old (he was mature when I got him) and the move was too much for him. I tried a very young adult female. She did great until the day I found her dead less than 2 weeks later. I next tried it with some fry. They were 2-3 weeks old. They did great for a week to 10 days but then one day they were all gone. Next I tried even younger fry, because there are some fry being born in the main tank as well and those that escape being eaten by the adults or being sucked into the filter seem to do okay. I figured maybe if they grew up with whatever conditions are causing the problem in the main tank, they would do better. Nope. A little over a week after introducing them, they're all gone. It's not that the conditions in the tank are just unsuitable for aquatic life. When I first set the tank up, I let it cycle then I put in some quarantined guppies from the store in to get it started. These have all done fine, and some later additions from the store have also done well. And like I said, I can move fish from one breeding/grow out tank to another freely with zero losses. It just seems that the move from the grow out or colony tank to the main tank is the kiss of death. They're not getting sucked up by the filter, either. The water tank parameters for the main display are: Nitrate - 5 ppm Nitrite - 0 Ammonia - 0 GH - 280 KH - 240 ph - 8 Temp - 77 (this fluctuates by less than a degree, I run my heaters on an Apex controller and can confirm no temp issues) I also test all the breeding/grow out tanks. They have the same stats except they have slightly higher nitrates, around 20, and slightly lower ph. Both differences are due to the plants taking up nitrates and co2 (more co2 in the breeding tanks = lower ph) but it's not really anything drastic. Actually the temps of the breeding tanks are all a little bit different, they are all running their own heaters and they can be a little fussy (those are not on the apex) For filtration I use some HOB and some sponge filters in the breeding tanks. For the main tank I use 2 seachem tidal 75's. I don't care for canister filters and these two seem to be doing the job. I am not trying to save every single fry that is born in the main tank, so the occasional fry getting sucked into the filter isn't a big concern. The tidal has multiple intakes anyway so it would be difficult to cover them even if I wanted to. Any thoughts?