you wont need to mess about cleaning the filter sponge yet, the longer the better and certainly dont replace them ( as reef one recomend) untill they are literally falling apart, all thats needed is a quick gentle squeeze in a bucket of water that you have taken OUT of the tank, so in effect dirty water
if the filter ( bubbles at the the top of the bubble tube) starts to slow down, you may find that you need a new air stone ( located under the filter housing)
im also thinking that your filter hasnt started to cycle with those readings( or you have done too many water changes without declor and wiped it out), use an API mater testing kit and test daily,
unfortuntaly you are going thru a fish in cycle, you may well lose your fish,theres a thread at the top of the main page here that includes facts on fish in cycling
or theres the alternative.. return the fish
with no readings it pointless doing water changes, so test everyday then as soon as you get an ammonia reading do a small water change, its a no win situation you are in as although nitrItes are deadly to fish, you cant complete your cycle without them
ammonia>nitrite>nitrate
i know you say you were given bad advice when you bought the biorb, but there is a booklet with biorbs, that states that
1 new fish should be added no sooner than 28 days after eachother till max stocking is met ( yes the filters are that bad)( which is never correct for the size, surface area or general suitabilty of these tanks) neon tetras prefer stable mature setups
