Hello, I am Dr. B, a licensed veterinarian and I would like to help you with today.
From your history, rather then this being an algal problem, it sounds more like you are seeing a bacterial bloom. This tends to arise on substrate when there is excess waste material in the water (ie uneaten food, decomposing plants, fish feces, etc). The reason you may be seeing more of this then one would with a traditional glass tank is because the liner material will be more porous the glass or plexiglass. This means the bacteria will be able to gain a foothold on this material and form ‘fronds’ rather then just floating in the water and causing clouding.
In regards XXXXX XXXXX the situation, I am glad to see you have adequate filtration and that everything is online there. We want that up and running but can also decrease the surplus bacterial population with a thorough regular hoovering/vacuuming of any gravel and the filter lining itself. Removing this material will decrease your bacterial population and given your filter a better chance against the rapidly multiplying bacterial population.
While decreasing the current bacterial population, we also need to decrease their food source (those aforementioned wastes). Therefore, we do want to make sure we are not overfeeding the fish (consider just feeding them until they are full or offering a few smaller meals a day to limit what feed goes to waste) and that the hoovering also includes any waste materials present. Any dying or questionable vegetation should be removed.
As well, if the situation is severe, then these tanks often benefit from routine partial water changes. Partial water changes benefit us in these situations in two ways. It is again a means of decreasing the bacterial population but also will help reduce any elevated nitrogenous wastes levels. In saying the latter, we do want to make sure to check your water’s current nitrogenous waste levels (especially the nitrites, nitrates, and ammonia, as elevation of these can harm our fish as well). If you find any issues with the water parameters, then this will dictate how much and how often a partial water change would be indicated here.
Finally, just in case this issue arose after you cleaned the filters, then we’d have to consider that this issue could have been precipitated by a loss in the ‘good bacteria’ of the biofilter. If this is the case, then while doing the above, we’d want to consider using a biofilter bolus (ie Tetra Safestart ) to support getting your biofilter back up to par in this tank (and thus removing nitrogenous wastes from the environment and from the blooming bacteria).
Overall, the material you are seeing is likely due a bacterial bloom. The key to tackling this situation is to decrease the bacterial population (via hoovering out this material) while removing the organic waste (their food source). While doing so, it would be prudent to check your water parameters and support your biofilter at this stage. These steps will ensure you are not missing any triggering issues with the tank/water health and make sure your biofilter is up to snuff to tackle any lingering nitrogenous wastes. Furthermore, if this is a recurring issue for your tank (which it may be if the bacteria are finding the liner to be particularly porous), then do make sure to adopt a good aquarium husbandry routine including routine water parameter testing, regular partial water changes and substrate vacuuming to avoid future bloom issues.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
Remember that if you have any lingering questions or concerns, please reply so that we may continue our conversation. I will be happy to work with your further and do everything I can to provide you with the service you seek. Please remember to rate my answer when you are satisfied (with4-5 stars or a happy face) so that I may receive credit for my assistance. Thank you & have a great day. : )