ok, not sure how much you know about water quality .some tips.
first off , the UVc lamp needs replacing every year ,the lamp will still light BUT won't give out UVc ,this will cause an algae bloom
all the sun we are having will also boost algae growth
High nitrates also boost algae growth
by "low plant levels" i assume you mean lilies and plants have not grown much yet ? when they grow,they use up nitrates
you don't say what the pond parameters are
do you know about nitrogen cycle ?
do you have a water test kit ? I check my PH every day
amonia levels will spike early in the season because filter bacteria is low,you can boost levels with additives
bicarbonate of soda brings up the PH level and helps create the structure for good bacteria ,along with a small quantity of calcium carbonate
probably lots of kit you don't have
up to 50% water changes can help but try to avoid rapid changes in temperature more than 3 degrees per 24 hours
rather than use "tap safe additive" which is expensive you can buy Sodium Thiosulphate Pentahydrate (penta) which is very cheap in comparison
as i said,if you have air pump it helps