Bristlenose Plecos are very hardy fish and given the various different stats on water parameters don't seem to be overly picky although the water Hardiness should be 10-15 (i assume ppm) - as I found in one article or 20-25 (I assume PPM) in another article. I have somewhat hard water and they have always done really well. The only time I've lost a pleco was 1. When they were very small - may have been sucked up during cleaning. 2. Once I found a baby partially eaten - my tanks only have peaceful fish but stuff happens. 3. General horrific water accidents (I'm ashamed to say I've had two where I lost about half a tank a fish within seconds after adding water for a water change. We still don't know what we did wrong - it's happened twice and each time I lost one or more of my plecos.
The riskiest time for fish is 1. Right after you buy them and 2. During/after a water change. You just had some bad luck. Pleco's especially like a fairly high flow of water so make sure you have an oversized filter for your tank (regardless of how many gallons they say they process it is very dependent on the amount of media you have in the filter because the media slows that rating down tremendously - so your filter may not be big enough. For example, even in my 29 gallon tanks I use a filter rated for a 100-120 gallon tank. It's a problem because my plecos are in with Gourami - two species with different requirements - Gourami's I believe, need softer water than I can provide, while I'm spot on with my plecos. Plecos love currents, Gourami love quiet waters. I have now turned down my filter to low to slow the water speed and instead added 3 small air stones at various locations and the pleco seem to like to hang out near them.
Plecos need lots of OXYGEN so the air stones will help a lot with that - and they like high water flow.
Even though your tank was cycled I think you should have waited a couple more days to add fish. I would also get Tetra's bottle of bacteria and add about half of it (diluted in quart or so of tank water) to your tank - you may have lacked enough bacteria to handle that many fish at once. Since I started doing that when I first set up a tank and later before adding fish I really don't need to go through a 30-60 day cycle - my water parameters are always perfect within a day or two of adding the bacteria. But even if the tank seems ready for fish - I always wait a couple of days and test each day to make sure the parameters hold in place. Then I purchase perhaps 2-3 fish and hope for the best- only once have I lost a new fish since I started doing that. I also have to adjust my PH down to 7.0 (with a PH regulator product). Our tap water is 9.4 I know that was the cause of deaths during my early fishkeeping experiences. NO freshwater fish can handle that high of PH. I now use this product (tiny bit in each bucket of water change water - I'm not dumping ph 9.4 water into a 7.0 ph tank. The bucket water is always already adjusted to PH 7 before I add it - I also declorinate each bucket - so I can't use the python water changer because there is no way to do that with that product (I don't know why everybody thinks that thing is so awesome when you can't treat your water BEFORE it goes into the tank.)
So my conclusion: not enough bacteria for so many fish and not enough oxygenation in the water (that's why people are suggesting plants - and that does help a lot but doesn't add to the water flow speed the pleco's favor. My current pleco's are very tiny (my last one were mature when we had that terrible accident - they were awesome fish). I'm getting 4 more small pleco's soon. In about 2 years they will all be mature and I probably will have to split them up from the Gourami (who probably are living too short of lives in my tank anyway- I need to change the water parameters more to their needs) plus then I can give the mature pleco plenty of water flow and oxygen, Again, good bacteria is your friend, but there is no way to measure how much you have so don't be afraid to add additional bottled bacteria to your cycled tank any time you are getting new fish just to make sure you have enough.
