custom filter tag I can't get to work

[Rookies DJ]

I create my own custom filter tags to using in my project but every time i put in my HTML it just product this errors Invalid filter:

I don't where i do wrong, I try putting the {%load tag%} with child templates, it didn't work 

I try name it differently 

@register.filter("multiply", multiply) @register.filter(name="multiply") 

@register.filter() 

didn't work either 

Here my HTML code that include the filter:

<div class = "float-right  my-4 chartjs-render-monitor" id="chartContainerPH" style="width: 49%; height: 400px;display: inline-block; background-color:#FDFDFD;">
   <center>
   <a class="title-link" href="{%url 'ph' %}">PH:</a>
   <p> {{tank_system.PH|latest}}, </p>

   </center>
</div>

and here my code for my custom filter

from django import template
register = template.Library()

@register.filter('latest', latest)
def custom_last(value):
    last = None

    try:
        last = value[-1]
    except AssertionError:
        try:
            last = value.reverse()[0]
        except IndexError:
            pass

    return last

my views

from django.shortcuts import render
from django.views.generic import TemplateView
from zigview.models import tank_system

from django.contrib.auth.decorators import login_required
import logging
logger = logging.getLogger(__name__)


try:
    @login_required(login_url='/accounts/login/')
    def index(request): #gose to main dashboard page
                tank = tank_system.objects.all
                return render(request, 'FrounterWeb/extends/includes.html', {'tank': tank})
except:
    logger.error('index page request failed/errors')

I intend to use two custom filter