Jonny, I think that the next purchases you should go after are a heater, agree with the titanium heathers (the brand name on these are "WON Titanium Heaters") and a powerhead to increase your flow. It is true that stomatopods are tough but on the same token so are alot of animals and that doesnt mean we should skimp out (your pretty adaptable to alot of nasty conditions, would you want someone to stick you in a substandard environment just because you could hack it for awhile?). Im not saying that you would stick your mantis in a terrible environment but its worth pointing out in my opinion. Hope that helps a little man.
Brian
Brian