I'm pretty sure they were common pleco's. I did remember that they had sunken bellies so something was probably wrong.
I just cleaned out the tank and letting it cycle again so I'm going to give it another shot but this time I'm going to try some oto's.
It could be a number of factors (like ph shock which Tolak suggested);
a. Ask the shop you get your fish from their PH; if the ph at the shop is very different from the ph in your tank, this can make it very difficult to acclimatise fish succesfully to a tank particually if they are not in the best of health (i.e. under weight, very stressed, sick etc).
b. Test your tank for ammonia, nitrites and nitrates- your current fish may be fine, but fish can become accustomed to things in a tank, but can take quite some time getting customed to for a new fish. If your nitrates are above 40, or you have any ammonia in the tank, you should increase how often you do water changes and things to the tank.
In a healthy tank ammonia and nitrites should always be 0, while nitrates preferably under 40.
c. You may not want to disturb your plants but they will be fine being temporarily uprooted and then replanted so you can clean the subtrate they grow on. Cleaning your substrate is very important for two reasons;
1. Gravel traps rotting waste very easily, wether it be dying plant matter or fish poop etc. Fish waste and rotting matter creates ammonia and can upset even the most established of filtration systems if not dealt with on a regular basis- filters will take out a great deal of waste, but cannot do the job by themselves entirely.
The filters main purpose is to grow benneficial bacteria on its sponge/s which breaks down toxins like nitrites and ammonia, but it can become overloaded if there is too much to deal with.
2. Sand can harbor anerobic bacteria and gases in it, which when left to build up in large quantities can leak out from the sand in bubbles and poisen the fish (note when you clean sand you sometimes see bubbles coming out of the undisturbed area's).
To prevent this happening on a dangerous level you should clean your sand regually- if you have both sand and gravel you will have to deal with both of these problems, so regardless of how much you love your plants in their current position you should clean your substrate at least once every 2weeks, but preferably once a week
.
Because plecos live right down at the bottom most of their time, you should keep your tank substrate especially clean. From a pleco or cory (or any other bottom dwelling fish) point of view, it would be a bit like us sleeping in a soiled bed and eating off dirty plates otherwise. Just like us, fish are far more likely to get sick from living in less than clean conditions (even if your water looks sparkling clean, there may still be issues with it or the tank).
A lot of people assume if a fish is hardy then all of its type will be hardy no matter what the situation or circumstances, but stressful conditions can wear down even the toughest.
Many fish that arrive in lfs (local fish store) or lps may have been up to 8hrs or more in transport constantly when they arrive at the shop, only to be quickly acclimatise and dumped in a small, overstocked tank.
They may be put with fish they are not compatable with, or deseases may already be spreading quickly in their numbers from the stressful conditions and crowding.
Many fish can often be half starved (particually smaller fish due to their shorter guts, or baby fish due to their high energy needs) on arival.
Some lfs only wait a couple of hours before they sell the fish after arival, where the fish could then be transported to the buyers tank in god knows what conditions or length of time. Sometimes, this is just too much for the fish and they die shortly afterwards or over a period of days.
You mentioned the plecs you bought had sunken bellies- this could be one of two things or both;
a. Many baby or half grown plecs are starved from transport when they arive at lfs, and usually do not have enough time to put on a decent amount of weight before being sold- stress and shock kills fish particually when their health isn't good.
b. There are plenty of deseases and ills that effect fish that can cause a sunken belly, but Internal Parasites is a nasty desease that causes this symptom in particular and is becomming increasingly more common in the aquarium hobby. It can be cured but only with medicines (particualy Levamisole) obtained by perscription from a vet, it is very rarely ever sold in petshops (not at all in the UK i think).
My advice to you is to clean your tank more regually and thoroughly and buy some accurate test kits for ammonia, nitrites and nitrates (not the paper strip test kits, but test tube ones as these tend to be more accurate) :nod : .
You should do a 20-30% water change or 40-60% water change (depending on what fish you have and how many and so forth) once a week (making sure to add dechlorinator/water conditioner), and clean the tank filter out once every 1-3 weeks (depending on the tanks stocking). You should clean your substrate once every 1-2 weeks either via a gravel vac/syphon or with your hands, a net and water changes(stirring up the subtrate, netting out any waste, and then doing a water change).
Thirdly, when you buy a plec you should ask what the lfs PH is as if it is very different to yours you will have to acclimatise the fish much, much longer and more gently (even then ph shock could still effect them). If their plecs don't look very well (underweight, pale in color, damaged fins, cotton growths, complete lack of activity or willingness to eat etc) you should eitheravoid them or ask your lfs to hold them for a week or so, feeding them well, before you get them (as they may just be stressed and under fed from transport).
How many gallons is your tank
?
You'd have to ask API or someone who knows alot about medications.
