Tips on reducing mulm

I have a low tech 55g planted tank which almost never receives a water change. Despite a low fish load and careful feeding, I've determined my lingering algae issue is due to mulm buildup.

Last couple weeks, I've vaccumed up the mulm, but don't care to due this as I've got flourite sand and it's hard not to get some sand with it. Also, when I perform these small (2-3gal) water changes weekly, is this, along with top-off water enough to cause algae producing co2 fluctuations?

Or...is adding more shredders the best option, and how many for a 55g? I have 6 fish, one of which is a chinese algae eater, and I think I still have two amano shrimp. I say I think I have them because they immediately went into hiding when i purchased them several months ago. I've rarely seen them and never in the area where the mulm tends to accumulate. Are they doing me any good? Should I just get more, and where to find them? Or snails...and how many?

Thanks for any advice.

Had an additional thought....what about oxygen? Could I help, or solve the problem with an airstone at night or could this just complicate the issue with possible co2 fluctuations?

 

I'm dosing 1/4 tsp of kno3, and 1/8 tsp kh2po4 and a capful of plant gro liquid ferts every Sunday. Light was the big issue, but I've raised my bulb as was recommended here. My 54w VHO t-5 bulb now sits 6 3/4" above the water. I think the best evidence was noticing how long my glass stayed clean and algae free after I had done a good bit of vacuuming which I had rarely done. So...I'm not certain, just my best guess based on what I've done and read.

I've worked hard at trying to reduce co2 fluctuations, even going to the extent of frequently topping off water in my uncovered tank to prevent dumping lots of co2 rich water in. If I do 50% changes, would I not be inviting algae?

I don't have any C. siamensis although I'm familiar with them. I'll go this route if this is what people recommend, just not excited about filling my tank with a bunch of bland looking fish which don't inspire me.