Cucumber is a fine food to feed plecos every now and then, i've feed my largest common pleco cucumber as often as a couple of times a week, what i mean with feeding cucmber too much is to feed it something like every other day or something like that. If you are just doing it a couple of times a week, once a fortnight or once a month etc you probably won't experience any problems with the pleco becomming dependant on the cucumber
.
Cucumber can make a good every now and then food for large plecos which have difficulty eating small algae wafers or in tanks where the owner suffers problems with other fish gobbling up the algae wafers before the pleco has a chance to eat them etc.
For young juvenile and mature large plecos though, particularly with common and sailfin/gibby plecos, a diet which focuses on algae wafers with occasional high protein food additions (like prawns, mussels, slices of raw fish meat, cockles etc) is the best diet for young plecos to help get the vitamins, minerals and protein they need in their diets to help them grow well
.
Plecos like sailfins and commons have a much more algae based diet when young, however as they grow larger they may find difficulty in getting all the protein they need from just algae and veg to continue growing well- many sailfin and common plecos naturally move onto a slightly more high protein (although still very veg-orientated) diet as they grow into large plecos.
With the fat fish problem, it could be fat for two main reasons- the first is simply constipation, pea's will help a great deal with this if you cut down on dried/high protein foods and feed the fish in question only cooked de-shelled chopped up pea's for a while until its digestive problems get better. (edit) If you have a fish/s suffering from constipation, you should review what exactly you are feeding your fish and how often and change their diet to help prevent the problem from occuring again in the future, because constipation for fish is a serious problem and if left untreated may prove lethal for the fish, since the food in the gut which is not being passed out will begin to rot in the fishes gut and this can cause a whole number of other lethal problems, like swimbladder disorder, dropsy, internal bacterial infection etc.
The second main cause of bloating is dropsy, which can be caused by numerous things. "Dropsy, is actually a "symptom" and not a disease in itself. The standing scales (although these may not present themselves as a symptom until the dropsy is a lot more progressed) and blown up appearance are the result of swelling within the tissues and organs of the fish due to a build up of fluids in the kidneys and an inability to excrete this fluid in a normal manner. In other words, advanced kidney dysfunction.
The problem with treating this disease is that there are a number of "potential" causes: bacterial infection, parasites, cancerous tumors, virus', internal fungal infections or sudden temperature drop of more than 3 degrees", more info read this link;
<a href="http/badmanstropicalfish.com/articles/article24.html" target="_blank">http/badmanstropicalfish.com/articles/article24.html</a>
.