Thanks, TP - I think Crusty died of bloat - At least that's the only thing I know of to call it (some kind of infection or blockage). I've heard that happens commonly with plecos. I'm still sad about it (he was 8 years old). He was the most interesting fish - he loved music, liked to be petted. I've thought that if I did have any more plecos I'd put sand in the tank since that seems to be more natural for them and I've read it helps their digestion. But I only want to adopt fish or any other animal, not buy them.
Ah the "bloat" sounds like Dropsy, dropsy is really a condition, a symptom, rather than a disease and generally stands for general kidney failure in the fish (the fishes osmotic processes go wrong and the fish starts to retain water inside its body, causing the bloated appearance).
The problem with dropsy is that it can be caused by numerous things, and even when you are sure of the cause of the dropsy it may not even be treatable either way- cancer, internal bacterial, viral or parasitic infections, water quality problems causing stress on the fishes body, constipation, contaminants/toxins or unwanted substances in the tank etc can all cause dropsy.
In the past people used to recommend adding salt to the water of freshwater tanks in the belief that it was good for the fish. Its true that pure/marine salt can cure certain infections or external parasites on fish, but long term usage of salt in freshwater tanks causes great stress to freshwater fish How tolerant the fish is of salt depends on the salt levels naturally present in its native wild habitat. For example, after thousands of years of floodwaters washing into the Amazon, the Amazon is almost 100% devoid of salt so a lot of amazonian fish cannot tolerate salt well since they have evolved in a pretty much salt-less habitat. A lot of varieties of Mollys (a livebearing fish) on the other hand though come from habitats that are coastal, where freshwater meets marine water, and mollys have actually evolved to be able to switch between freshwater, brackish and even marine habitats- most varieties of mollys, although commonly sold as freshwater fish, actually benefit from having a little bit of salt in their water.
Either way though, adding salt to a tank where a fish is suffering from dropsy usually makes the dropsy 10 times more lethal.
With the sand thing, it won't help the pleco's digestion, but it is still very beneficial for pleco's for other reasons. Gravel is notorious for trapping waste in it, and this decaying matter can harbor nasty diseases like columnaris or simply create ammonia problems in the tank (which may not be imediately picked up at first since ammonia is more concentrated at the bottom of tanks) which is bad because ammonia is toxic to all aquatic life etc.
Sand is much better though on the other hand in comparison to gravel because waste does not trap easily in it, rather it just sits on top of the sand where it can then get swept into the filter by the current. Since pleco's are high waste producing fish its good to have a sand substrate since sand substrates generally stay cleaner, which is obviously better for the pleco

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Sand is also good for a lot of other fish too though, mostly fish which feed with barbels/whiskers around their mouths, as most fish which have barbels to help them feed (like corys and numerous types of loaches) have evolved in habitats that have sand or fine mud as a natural substrate so find it a lot easier to feed off sand than gravel. Rough/sharp gravel can also damage the barbels/whiskers of certain types of fish, leading to painful bacterial infections in the fishes barbels which erode them away- fish with barbels are a lot less likely to get such bacterial infections though if they are kept in a tank with a sand substrate though

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If you ever think about getting back into fishkeeping, i would recommend checking out this fish forum below;
Tropical Fish Forums
I've been a member there for years now and it is a really friendly and informative forum to be on, i've learned so much about fishkeeping since joining there years ago, whenever i've encountered problems in fishkeeping i have always been able to find the right information and help i need to help my fish and to ensure they stay healthy and lead a good quality of life and more etc

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