It does sound like a bacterial infection, when you say you changed the water, how much did you take out exactly and did you dechlorinate the new water? Meth blue kills off beneficial filter bacteria (although its a very good powerful med none the less) so you need to test your water for ammonia, nitrites and nitrates to see what state the water quality is in. How long, wide and high is the tank as well (or how many US or UK gallons/liters is it)
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Mixing male betta's with females ones, or males betta's with guppys, is not advisable as they are solitary and territorial fish by nature and can turn very aggressive towards brightly colored small fish like guppys (male betta's can view colorful guppys as rival male betta's) and they don't like staying with female betta's as in the wild these two fish only come into contact with each other when they want to breed, and otherwise stay out of each others way when they don't want to breed. Female betta's are not as aggressive or territorial as the males, but they can still be grumpy with other fish and their own kind at times, which is why its adviseable to keep them in groups of 5+ (so if there is any aggression amongst the females, its more likely to be spread out amongst them than concentrated on one particular female).
Cherry barbs, neons and most other types of small tetras and barbs are all shoaling fish, meaning that they thrive best when kept in groups of at least 5-8+ of their own kind- shoaling is a defensive behavior in such fish, and if the fish aren't keep in larger enough numbers of their own kind to shoal properly, it can leave them feeling stressed even if there is no apparent threat in the tank. So once you have finished sorting out your fishes health problems, it would be best to gradually increase the numbers of your individual types of shoaling fish- if increasing their numbers to ideal levels means that you will over-stock the tank, it would be best to re-home one or two of the types of shoaling fish you have.
With the guppys, if you are keeping different genders of guppys together, then ideally you should have 2-3females per male, as male guppys have ver high sex drives and if there aren't enough females to go around, they may over-harass one particular female, making her feel very stressed. So its best to ensure you have plenty of females when keeping mixed gender groups of guppys
. Be warned though, these fish can breed very easily and a single female can produce up to 30fry or more a month, however on the plus side though unless you do anything in particular to protect the fry from the adult fish, most fry (if not all) will get eaten sooner or later- however this depends on many factors, like what types of fish you have in the tank and how much planting you have etc. Sometimes masses of fry survive, which makes the tank over-stocked, so be careful.
Also its important that you ID what sort of pleco you have- there are hundreds of types of pleco's, and unfortunately some of the most commonly sold ones are some of the largest growing ones and can grow up to 1-2ft long. Pleco's can also vary massively in what sort of diet they thrive best off (some need rather meaty foods, while others are almost completely herbivorous etc), so that is another important reason why you need to get an accurate ID on the pleco you have
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Does your pleco look like either of these two in particular;
Sailfin pleco;
<a href="http
/www.planetcatfish.com/catelog/speci...?species_id=148" target="_blank">http/www.planetcatfish.com/catelog/speci...?species_id=148</a>
Common pleco;
<a href="http/www.planetcatfish.com/catelog/speci...p?species_id=88" target="_blank">http/www.planetcatfish.com/catelog/speci...p?species_id=88</a>
Or does it look a little bit or a lot different from these two pleco's?
At some point soon you should start to change your fish stocking, adding more or taking away/rehoming certain types of fish, a lot will also depend on how large the tank is as you don't want to over-stock it with fish (if you do it can lead to water quality problems or diseases and parasites running rife through the tank etc)
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) . It smells horrible, and tints the water brown, but I've had good luck with it. This stuff also doesn't interfere with the bacterial nitrogen cycle, so your bacteria should be fine.
