hmm I am not too sure about a cucumber.. they always make me think twice.. but anyway. a high level of ammonia.. did anything die recently or "disappear"? Also what is your feeding rate... its funny because I sued to think it was common knowledge to feed fish everyday, but as I went along the aquarium route I realized this is completely a myth.
What I forgot to ask, and is really important to know is what clean up crew do you already have? Like how many hermits, shrimps, snails, crabs, etc?? Because you have a fish only tank it is more often suggested that you obtain a skimmer, but they are big bucks in most cases and spending the money isn't an easy thing to do. You could also try to run GAC
, which does a fraction of what a skimmer does, but it is better than nothing, but do becareful with what you put in the water chemical wise only because it may get filtered out by this...